trigonometry

Depends on the statement of the task. However the most widely spread way is to use the Pythagoras theorem or the sine theoreme, you also shoud have the definitions of sine, cosine, tangent in your mind all the time. Or and I forgot Molhweide formulas as well as the tangents theorem. Apart these general paths there can be specifical ways to solve a triangle.

Simpliest way is by sin,cos,tan.
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-The hypotenuse of a triangle is the longest side of it.

sin(a) = the length of the side opposite to angle a divided by the length of the hypotenuse
cos(a) = the length of the side next to angle a divided by the length of the hypotenuse
tan(a) = the length of the side opposite to angle a divided by the length of the side next to angle a

after solving the value of sin/cos/tan a (any of them) you use the respective function asin/acos/atan to the value -> returns the angle

Notice: some systems return the angle in an unit called Radian instead of Degree, this might make the results seem way too small.
The conversion from Radian to Degree goes -> deg = rad * 180 / pi