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)
