EXPLANATION
Let's see the facts:
-One angle = 77
Since this is a right triangle we know we have a 90° angle.
We also know that the 3 angles of a triangle add to 180°.
-Right angle = 90°.
The third triangle will be:
77+90 + third angle= 180
Adding similar terms:
167 + third angle = 180
Subtracting -167 to both sides:
Third angle = 180 -167
Simplifying:
Third angle = 13°
Answer is 13°.