Okay!
All we need to do to solve for x is to set up an equation!
the equation 3x + 4 represent the number of degrees of the bottom angle
→To set up the equation, however, we must find what the bottom angle is!
→This is easy since we know that all triangle are 180°
→The diagram already gives us to angles, so to find the third angle, we use simple subtraction
*the little corner with the red square is always 90°
180 = 90 + 50 + ?
? = 180 - 90 - 50
? = 40°
The angle is 40° !
→Now we can finally find x!
→To find x, we just set the equation they give us equal the degrees of the angle that we just found
3x + 4 = 40°
SOLVE !
3x = 36
x = 36/3
x = 12
We did it! Hope I helped ! :)