There are many formulas to finding a side of a triangle.
Be sure to understand the differences between a right-angled triangle and an irregular-shaped triangle.
Right-angled triangle
Two sides must be known.

where 'a' squared is the hypotenuse of the triangle.
To find the non-hypotenuse side, the formula can be rearranged to

'b' can be any non-hypotenuse side.
Non-right angled triangle.
Cosine rule
Two sides and an angle must be know.

That is just a few of the formulas I have listed.