a) Solve y=f(x) for x.
This instruction just means to manipulate the equation so that x is all by itself on one side. We can use a mixture of addition/subtraction and multiplication/division to do this with any equation.
f(x)=-3x+7
y=-3x+7
y-7=-3x
(y-7)/-3=x
b) Find the input to f(x) when the output is -8.
This means that it wants us to find the x value that provides a y value of -8. To solve this, we just need to plug -8 in for y and solve for x.
y=-3x+7
-8=-3x+7
-15=-3x
x=5