Since this is a right triangle, you can utilize the Pythagorean Theorem.
In THIS scenario, your hypotenuse and one of your legs are given. Therefore,
a=?
b=12
c=13
Now, substitute those values into the Pythagorean theorem and simply solve for "a"
Remember that you cannot have a negative measure. You cannot have a length measure -5 units. Therefore, we can ONLY take the positive value.
Thank you and I hope this helped.