First find the last angle of the triangle on the left
54+47+x=180
101+x=180
x=79
This angle, 51 degrees, and the bottom left angle of the second triangle should add to 180 degrees (straight angle)
79+51+x=180
130+x=180
x=50
The angle we just found, 40 degrees, and the missing angle should add up to 180 (interior angles of a triangle)
50+40+x=180
90+x=180
x=90
Final answer: 90 degrees