How can I find the length of the missing side of the triangle?

For a right triangle.

C^2 = a^2 + b^2

Assuming you want the hypotenuse...,

c = sqrt(a^2 + b^2)

And the other sides are also algebraically manipulatable

For oblique triangles trig works.

Law of Sines.

sin(alpha)/a = sin(beta)/b = sin(gamma)/c

Law of Cosines.

a^2 = b^2 + c^2 - 2bc cos(alpha)

b^2 = a^2 + c^2 - 2ac cos(beta)

c^2 = a^2 + b^2 - 2ab cos(gamma)