Oh, this is really easy!
Okay so the formula for the pythagorean theorem is a²+b²=c²
So it doesnt really matter whats a or b
So take 21²+15²=c²
That ends up being 441+225=c²
so next take 441+225 and that equals 666 then you have to find the square root so the square root of 666 is 25.81
S the answer is 25.81