Since we can find that the graphs Y value is by a factor of 2 looking at the graph we can find
f(3)=8

You are taking two times a number, which is defined as x. That is equivalent to 2x. Then, you are adding three more to that product.
Answer:
The domain is all of the x values. From the graph, all of the x values are greater than or equal to 0 so the answer is x ≥ 0.
F(-8)=8^2-3(-8)
=64+24
=88
Hope this can help.