Question

Three times the square of a non-zero number is equal to nine times the number

Answer 1

$D:x\neq0\\\\3x^2=9x\ \ \ |divide\ both\ sides\ by\ 3\\\\x^2=3x\ \ \ |subtract\ 3x\ from\ both\ sides\\\\x^2-3x=0\\\\x(x-3)=0\iff x=0\notin D\ or\ x-3=0\\\\\boxed{x=3}\in D$

Answer 2

3 x 3^2 = 9 x 3
3 x 9 = 9 x 3
27 = 27

Your number is 3