So you do a^2+b^2=c^2 so hypotenuse is the longest long so it is c and other leg can be b or a it doesn’t matter as long as you have 5 for c so a^2+4^2=5^2
a^2+16=25
I squared the numbers 4times4=16 and 5times5=25
Now you minus 16 because we are trying to figure out what “a” is or other words leg two
So 25-16=9
Now a is still square so be got to take the square 2 away and square 2 is also know as square root so a= 3 the other leg is 3
Check
3^2+4^2=5^2
9+16=25
25=25
