Answer: x is 5
Step-by-step explanation: These two triangles are similar (meaning same shape but different size.) To find x, we must first decrease the scaling. The original triangle is 3 and x sides. Being scaled, it is now 12 and 20. 12 divided by 4 is 3, so that's the first side, and being equivalent, 20 divided by 4 is 5, so therefore the triangle has been scaled by 4 units. Therefore x side is 5.
