we can use Pythagorean's theorem for this one. let's just call x the missing side length.
21^2 + 28^2 = x^2
441 + 784 = x^2
1225 = x^2
x = 35
so the missing side length is 35.
by the way, the Pythagorean's theorem is \(a^2+b^2=c^2\).
hope this helped! please let me know if you are still confused :)