Question 653229
if f(x)=2x and g(x)=x-4 what is the value of f[g(3)]?

f[g(3)] says "f at g at 3".  More specifically, it reads, "evaluate g if x is 3, then stick whatever number you get into the x in the f function".  Always work inside to outside in these functions within functions.

So:

g(x) = x - 4
g(3) = 3 - 4
g(3) = -1

Now put -1 into f.

f(x) = 2x
f(-1) = 2(-1)
f(-1) = -2

So f[g(3)] = -2