Hello!
a. 10
b. 1
c. -11
To evaluate f(x) at a certain x, you simply substitute that value of x into the equation for x.
For example:
a. f(x) = 2x, find f(5)
Plug in 5 for x into f(x):
f(5) = 2(5) = 10.
b. f(x) = 4x + 5, find f(-1)
f(-1) = 4(-1) + 5 = 1
c. f(x) = -3x - 5, find f(2)
f(2) = -3(2) - 5 = -11