Answer:
15
Step-by-step explanation:
Using the Pythagorean theorem, or knowing that pythagorean triangles are can be scaled up/down, we know that this triangle is similar to the "3 4 5 egyptian triangle" Therefore x would be 15, as the sides are 3x the sides of the 345 triangle. we can prove this with sqrt(81 + 144) = sqrt(225) = 15
