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