use what you know of the angle of triangles to find x

If the right angle = 2x, then x = 45 degrees.
left acute angle = x = 45 degs
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That makes the sum of the angles
90 + 45 + 51 = 186 degrees
That won't work. 

The triangle goes as 
right angle = 2x
top acute angle of triangle = 51
left acute angle = x 

Triangle Angle Sum Theorem: Angle A + Angle B + Angle C = 180⁰

Angle 1 = 90^o

Angle 2 = 79^o

Angle 3 = 180 - 90 - 79 = 11^o

Angle 3 is x, as the other 2 angles are given.

Therefore: x = 11^o