Answer:
x = 16
Explanation:
Use that the addition of all internal angles of a triangle must add up to 180, and the fact that the two given triangles are similar:
51 + 65 + 4 x = 180
combine and solve for "x"
116 + 4 x = 180
4 x = 180 - 116
4 x = 64
x = 64 / 4
x = 16