well I do not see the figure but I will assume it is a triangle. there are 3 angles in a triangle. Those 3 angles add up to 180. Since we know this, you can use the equation
angle1 + angle2 + angle3 = 180
we have 2 of the angles so with a little algebra we can solve for the last angle like this
180 - angle1 - angle2 = angle3
plug in 2 angles
180 - 50 - 90 = angle3
So answer is
angle3 = 40 degrees