The hypotenuse right triangle is 26 mm one leg of the triangle is 10 mm what is the length of the other leg

Hypotenuse is 26 mm
so if we use the p theorem
a^2+b^2 = c^2
and theyre giving us c (26mm)
and theyre giving us one leg (10mm)
just plug it in and solve for the missing variable
10^2 +b^2 = 26^2
100+b^2 =676
b^2 = 576
square root of both sides to solve for b
b = plus or minus 24
But the length cannot be a negative length
so the answer is 
b = +24

Think of it as five or above give it a shove. change the 5 or above to the right to a zero and add one to the number on the left.