ok so
for a right triangle with legs legnth a and b, and hypotnuse legnth c
a^2+b^2=c^2
hypotonuse is 13
one leg is 12
other leg is ?
a=12
b=?
c=13
sub and find ?
12^2+?^2=13^2
144+?^2=169
minus 169 both sides
?^2=25
sqrt bot sies
?=5
the legnth is 5 units