Answer: [tex]10x^2+5[/tex]
Work Shown:
[tex]f(x) = 2x+5\\\\f(g(x)) = 2(g(x))+5\\\\f(g(x)) = 2(5x^2)+5\\\\f(g(x)) = 10x^2+5\\\\(f \circ g)(x) = 10x^2+5\\\\[/tex]
Notice how I started with the outer function f(x). Then I replaced every x with g(x), followed by plugging in g(x) = 5x^2.