Answer:
9
Explanation:
we can use the pythagorean theorem so (15)²= (12)² + (x)²--> (15)²-(12)²=x²
take the square root of both sides ans x=9
note! this is a special type of triangle called a 3-4-5 special right triangle because all the sides when divided by a factor of 3 will simplify down to the sides 3-4-5 (only true when the hypotheneuse is the 5 because it's the longest side of a triangle)