R1 = StrToFloat(Edit2->Text);   //R1 in Ohm
  R3 = StrToFloat(Edit1->Text);   //R3 in Ohm
  V2 = StrToFloat(Edit3->Text);   //V2 in miliVolt
  V1 = StrToFloat(Edit4->Text);   //V1 in miliVolt
  Av = (R3/R1)*(V2-V1); //Av=(Rf/R3)*(V2-V1)

Offer :

2N3055 Power Transistor NPN 15A 100V 2Pcs	Through Hole Right Angle Power Socket 2.1mm 5pcs	10 Ohm 10W Resistor 2Pcs	1N5407 Power diode 1000V 5A 5Pcs	2200uF 35V Power Capacitors 5Pcs
LM338 TO-220 2Pcs	Car Fuse Blade Mini/Standard 5-30A 32V 5Pcs Per Lot	LM2596 Switching Regulator Module 1Pc	Microchip 16F877 Development Board 1Pc	Power Transistor TIP42C 5Pcs 6A 100V

Offer :

2W Power Resistor With Various Value 22R-1M, 5Pcs Per Lot	High Voltage Mylar Capacitor With Various Value 100pnF-22nF, 5Pcs Per Lot 1-3Kv	Power Amplifier Transistor Toshiba C5198 140V 10A	Molex Connector Male 5 Pcs, Various Model from 2 to 5 pins	Grinder Stone With Various Shape 1Pcs
Power Amplifier Transistor Toshiba A1941 140V 10A	Packaging Bubble Padded Plastic Bag Various Size , 5Pcs Per Lot	SMD Power Diode With Various Value M1,M2,etc 10Pcs Per Lot	Resistor 1/2 W 1% With Various Value 20R-1M, 10Pcs Per Lot	HDMI Connector 2Pcs
BTA41 800B 800V 40A TRIAC 2Pc	DS3231 RTC Module 1Pc	Capacitor 6.8uF 250V-Crossover-Tweeter-Non Polar 1Pc	DPST Switch 250VAC 30A 125VAC 35A 1Pc	BTA16-600 TRIAC 5Pcs
Viper 12A Low power OFF-line SMPS primary switcher DIP8 5Pcs	Super Capacitor 15F 5.5V 1Pc	Super Capacitor 1.5F 5.5V 1Pc	1mR Resistor 10mmx2mm 2Pcs	Super Capacitor 4.0F 5.5V 1Pc
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