Evaluate this function when f(x) x=3
f(x)= 4x+6
the answer is 10 but i don't know how it got this.

Evaluate this function when f(x) x=3
f(x)= 4x+6
Important! The "x" is just a place-holder! And "f" is just a name.
Just replace the variable "x" with "3":

FOR x = 3
f(3) = 4*3+6
f(3) = 12+6
f(3) = 18

The answer is 18 NOT 10 (as you said).