Question

Leg =7, leg =24 what's the length

Answer 1

(assuming this is a right triangle)
This is a Pythagorean triplet.
The hypotenuse is 25.

7, 24, 25

You can solve this using the Pythagorean Theorem, a^2 + b^2 = c^2
Where a and b are the legs and c is the hypotenuse, or longest side. 
We have that the two legs are 7 and 24. Plug these in for a and b.

Plug in values 7 and 24 for a and b.
7^2 + 24^2 = c^2

Simplify 7 squared and 49 squared
49 + 576 = c^2

Add the numbers
625 = c^2

Square root both sides
25 = c

So the hypotenuse, or the third side, is 25. :)<span />

Answer 2

You would use Pythagorean theorem. 

7^2 + 24^2 = h^2
49 + 576 = h^2
625 = h^2
25 = h

Therefore making the length 25