Using the Pythagorean Theorem, we can figure out the missing leg.
If you don't know, the Pythagorean Theorem states that for any right triangle the sum of the squares of both of the legs is equals to the hypotenuse.
Where the variables a and b are the legs and c is the hypotenuse.
The variables a and b are interchangeable but not c.
Plug in the values that we know.
So, the other leg is equal to the square root of 39 units.