Help and explain !!!!!!!!!!!!!

Answer:
6x - 15
Step-by-step explanation:
f(x) = 2x + 3 and g(x) = 3x - 9.
[tex]f(g(x))=2(3x-9)+3\\f(g(x))=6x-18+3\\f(g(x))=6x-15[/tex]