Question

The triangles shown are similar.

Answer 1

$\dfrac{x}{9}=\dfrac{5}{3}\\ \dfrac{x}{9}=\dfrac{15}{9}\\ x=15$

Answer 2

15, because 3 goes into 9, 3 times and, 4 goes into 12, 3 times therefore x has to follow this rule because these 2 triangles are similar and because it already gives you 5 just take 5*3=15 there is your answer enjoy!=)