Need help in designing a logic circuit

I am desiginin a logic circuit using mentor graphics.ive done one part of my design but stucked at this part:
i wan this part to receive 2 sets of 2's complement number and keep substracting it, this is because one of the 2's complement number will change.. take the result(A) of the subtraction and multiply with 4/5(B) at last compare the result(A) of the subtraction and compare with B.

i cant think of any components that can actually do multiplication, hence i thought of using adders, shifters and decoder for the above but it is goin to be very tedious...

anyone out there can give some comments what component or what circuit to perform MULTIPLICATION for 2's complement binary number?

thanks