Verified answer

Here's my attempt

(f .g) x is called composition of functions or otherwise function of a function....here f(g(x))

easy way to handle this: u should start from the last function which is g(x)

g(x)=5-x

now look at f(g(x)).......instead of x u have 5-x

so substitute 5-x in place of x in f(x)

therefore f(g(x))=3(5-x)^2

Was i clear enough?do let me know:-)