EXPLANATION
Given the function f(x) = x^2 -5x + 1
and the function g(x) = 2x + 13
For g=2x + 13 substitute x with f(x) = x^2 -5x + 1
=2(x^2-5x+1) + 13
Expand 2(x^2-5x+1)+13
Distribute the parentheses:
=2x^2 + 2(-5x) + 2*1
Multiply the numbers: 2*5 = 10
=2x^2 -10x + 2*1
Multiply the numbers 2*1 = 2
=2x^2 -10x + 2 + 13
Add the numbers
=2x^2 -10x + 15
The value of (g•f) is 2x^2 -10x + 15