Question

Find function value f (6) if f (x) = 4x

Answer 1

Finding value of fuction is not very hard.
If we have function f(x)=4x and we want to find value of f(6) it means that to find this value we just have to put 6 instead of x.
Lets do this
f(x)=4x
f(6)=4*6
f(6)=24 - its the result.

Answer 2

Nice.
f(x)  = 4x.

f(6) = 4(6)    We simply substitute x = 6.

f(6) = 24.