I need help with question a and b. For a i know that gain =-R2/R1 and i know you just assume a resistor values so 5kohms and 1kohms for R2 And R1 respectively. What i dont understand is why you can just ignore the R3 and R4 resistors. Never really got the whole concept of op amps. And for question b im not sure how to find R3 and R4 by using the op amp. Thanks
In an ideal opamp with negative feedback, the two inputs are forced to be the same voltage.
As written for example in The Art of Electronics: "The output attempts to do whatever is necessary to make the voltage difference between the inputs zero."
Given this rule, you can calculate the values for your dc biasing circuit.
edit: Question a is doubtful as with switch S1 open, you can not have any gain (because you input signal is not connected).
R3 and R4 are biasing resistors to establish a suitable DC operating point - they serve the same purpose as the voltage divider for transistorized gain stages. Here, such biasing is necessary because your opamp is powered with one single voltage source only.
But don`t forget that the DC voltage produced by R3 and R4 will appear at the opamps output - amplified by the factor (1+5) if SW1 is closed. If SW1 is open we have unity gain feedback only.