#27 requires you to interpret a function graph
(a) f(1) = -2
to see that, start at the origin and move right to x = 1 on the x-axis ... from there, move down until you reach the function curve. Note that you're at the point (1,-2) which means f(1) = -2.
(b) f(x) = 2 at x = 4
to see that, start at the origin and move up to y = 2 on the y-axis ... from there, move right until you reach the function curve. Note that you're at the point (4,2) which means f(4) = 2.
When graphing a function, you are graphing the coordinates $\big( x,f (x)\big) $.
So, when trying to find the value of $f (x)$ for specific values of x, follow the instructions. You are basically looking for the y-coordinate. This is one of those times in math where it is difficult to explain and easier to learn by doing. If you try many examples, hopefully it will become clear what you are doing.