All of these involve simple substitution. Just replace the variable with the actual value.
8z + 3 >>> 8(8) + 3 = 64 + 3 = 67
3(7+ x^2) >>> 3(7 + 2^2) = 3(7 + 4) = 3(11)=33
s - 5t + s^2 >>> 4 - 5(-1) + 4^2 = 4 + 5 + 16
Get the trend? I am not sure I get that last one, is it:
x - y*3^2?