You must know first what kind of triangle it is. If the triangle is Right triangle well there will be solving for the opposite angle, adjacent angle and hypotenuse.
You can also use the pythagorean theorem
Finding c c²=a²+b²
and many more
you can use the SOH-CAH-TOA.......Sin theta equals opposite angles over hypotenuse and Cos theta equals adjacent angles over hypotenuse and Tangent theta equals opposite angle over adjacent angle.
YOU CAN ALSO USE THESE:
The inverse of sin theta is Cosecant theta.
The inverse of cos theta is Secant theta.
The inverse of tangent theta is Cotangent theta.