f(x)=2x+3 , f(2x+3)=5x. What is the value of 3x^2

Answer 1

$f(x)=2x+3$

$f(2x+3)=2(2x+3)+3=4x+6+3=4x+9$

Since f(2x+3) also equals 5x:

$5x=4x+9$

Solve for x, then plug that value of x into $3x^2$.