Easy,
the Pythagorean theorem is:
\(a^{2} + b^{2} = c^{2}\)
The a and b, being a value that you HAVE
and
c, being the value you are trying to find.
Example: have 1,2 for a triangle what is the third side?
\(1^{2} + 2^{2} = c^{2}\)
1+4=\(c^{2}\)
5=\(c^{2}\)
Then, just square root to find c.
\(\sqrt{5}\)=c
2.2≈c