To find f(8), look at each of the f(x) function and find x = 8. The y value is the answer you will plug in for f(8). The same goes for g(-7), but you will be looking at the g(x) function at x = -7.
f(8) = -4
g(-7) = -8
Make sure you solve in the correct order using PEMDAS.
Part A = She incorrectly add 1 (4/4) with 3/4, so instead of getting 7/4 she got 8/4. Part B=She needs 28/4 (7) cups for 4 batches because 4/1x7/4=28/4.