Lets get started :)
f(x) = x² + 6x
g(x) = 5 - x
f(g(x)) → We subsitute g(x) for x in f(x)
↓
= ( 5 - x )² + 6( 5 - x )
↓
Open Brackets
↓
= x² - 10x + 25 + 30 - 6x
↓
Combine like terms
↓
= x² - 16x + 55
f(g(9)) → We use the formula we found above and substitute 9 for x
↓
= (9)² - 16(9) + 55
= 81 - 144 + 55
= -8
