R3 = CDbl(TextBox1.Text)
        R1 = CDbl(TextBox2.Text)
        V2 = CDbl(TextBox4.Text)
        V1 = CDbl(TextBox3.Text)
        Av = (R3 / R1) * (V2 - V1)  'Av = (R3 / R1) * (V2 - V1)
        Label13.Text = Av

=========================================
Other items to offer :
Buy local , helps Australian grows

Lazer transmitter Module 1Pc	LM2596S ADJ DC-DC Step-down module 5V/12V/24V adjustable	0805 Capacitors with various value from 4.7pF to 1uF 10pcs /lot	Blue LED Crystal Clear 10Pcs	SMD RGB LED Module 1Pc
Through Hole Push Button 10Pcs	7 Color Flash Module 1Pc	RGB LED Module 1Pc	Reed Switch Module 1Pc	470uH 3A Power Supply Inductor 1Pcs

Offer :

3300uF 25V Power Capacitor 5Pcs	Electrolityc Capacitors with Multiple Values 0.47uF to 470uF	Small Signal Transistor With Various Value TO-92 Package , 5Pcs Per Lot	1 Watt Zener Diode with Various Value 5Pcs Per Lot	Green Mylar Capacitor With Various Value 1nF-470nF, 5Pcs Per Lot
KBP307 Diode Rectifier 3A 700V 2100VA Max 2Pcs	Ultrasonic Speaker 2pcs	Low Voltage FET 2N7000 5 Pieces	10 Pieces 2N3904 Transistor	CR2 Battery for Photographer
STEM UNON 80mm + - 7 degree 28.6mm steer 31.8mm handlebar 1Pc	Arduino UNO R3 CH340 1Pc	Arduino Nano 3 Brand NEW 1 Pcs	AD8232 ECG Sensor for STM32 Arduino with ECG electrode 1Pcs	Arduino Mega 2560 Brand NEW 1Pc
XR2206CP DIP CHIP 2Pcs	16 Melody Bell Kit Module 1Pc	TL494CN OP AMP 5Pcs	Green Polyester Film Capacitor 68nF 5Pcs	34063 Inverting Regulator,Buck,Boost, Switching CHIP DIP 8 5Pcs
Handle Bar Tape Blue-Army-Soft 1Pair	FM Receiver Transmiter Module DIY Kit 1Pcs	Digital Clock Kit 2.54cm Digit STC15F204EA MCU 1Pc	Super Capacitor 15F 5.5V 1Pc	Magnifying Glass Hardware Debug 40X 1Pc
CD4017 Decade Counter CMOS 5Pcs	High Voltage Capacitor 82uF 400V 2Pcs	N-FET 15N60 15A 600V	Red Fuse DIP PCB Mount 3.15A 5Pcs	74HC14 DIP14 HCMOS HEX INVERTER GATE 5Pcs
Electrolytic capacitor 0.47uF 50V 5Pcs	Electrolytic capacitor 0.1uF 50V 5Pcs	Power Transistor E13003 Vcb 700V 5Pcs	ULN2803APG Darlington Transistor Array 5Pcs	ULN2804APG Darlington Transistor Array 5Pcs
Stainless Disc 50x1.3mm 1Pc	ATMEL EEPROM I2C 24C512 5Pcs	ATMEL EEPROM SPI 93C46 5Pcs	XR2206CP DIP16 2Pcs	Bridge MB10M Rectifier 5Pcs 1A 1000V
Voltmeter Multimeter Lead 1000V 20A Max 1Pc	Pearl Izumi IQ XL Cycling Glove-Yellow Black 1Pair	High Voltage Capacitor 150uF 400V 2Pcs	CD4518 Dual 4bits Counter CMOS 5Pcs	HRM Soft strap for Garmin Magene,etc 1Pc