If given the length of two sides of a triangle what is the length of the third side

For triangles all three sides need to equal 180
Here's an example:

Lets say you have a triangle

One side equals 90
The second side equals 45
And you're missing the third side so replace the third side with x

Now you know that all sides MUST equal 180

so add the two sides you have (90 + 45) 
Which is 135

Now take 135 and subtract it from 180 
180 - 135 = 45

Now to check add 45 + 45 + 90 
which equals 180

So the missing side which is x will be 45
So x=45

When finding the length of the third side of a triangle after have been given the length of the first two sides, use the Pythagorean Theorem.
Example:
A triangle has a leg of 5 in and a leg of 7 in, Find the hypothenues
The formula is
c^2 = (a^2) + (b^2)
So
c^2 = (5^2) + (7^2)
c^2 = (25) + (49)
c^2 = 74
Find the square root of 74
c = 8.602