It depends on what other information you have.
If you have the lengths of both the other sides, use Pythagoras.
If you have one side and one of the acute angles, use the sine (or cosine) ratio.
There are other methods when other bits of information are available.
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by finding out the hypotenuse of the triangle
A right triangle with a hypotenuse of length 15 and a leg of length 8 has an area of: 50.75 units2
Hypotenuse = 24
The hypotenuse is 30.
That is it. Just the hypotenuse - provided you have the correct triangle.