question: ℎ(𝑥)= −2𝑥 +20. calculate

please give me a step by step direction on how to approach this. 

x = 9

is it h(x)= -2 * 9 + 20?

or h (9) = -2 * 9 + 20? 

for b). h(-2) ... does this mean h(-2) = -2 * -2 + 20? 

i need answers to these questions and a direct way for me to approach these problems. 

i really need someone to take the time to explain and break down this so i can read this and know how to solve function problems like this. i can solve something simple like -2*9+20. i cant figure out things like question 3 how exactly i am supposed to solve this.

please check attachment, and please help me out with clear detailed instructions. i purposely provided an easy question so someone could give me the help my teacher was unable to. 

thanks.