a = 8x5^2 -(3x4)^2 +7x2^3

b =(2x9)^2+3x(-5)^3 +2x3^2

c = -5^4 -3x(-5+2)^3 -(7x2)^2

The thing that you need to do is order of operations so that it becomes easier to work with. For the first problem

a = 8x5^2 -(3x4)^2 +7x2^3 get rid of the parenthesis parts first so:

a=8x5^2-12^2+7x2^2 then work on the squared part

a=8x25-144+7x8 remember that a negative squared is positive but since it is in parenthesis the negative will remain once the square is taken then do multiplication or division

a=200-144+56 then add or subtract in the order it is set

a=112

I hope this helps with the other two