Answer:
36 millimeters
Explanation:
From Pythagoras theorem, the square of the hypotenuse is equal to the sum of the square of the two other legs
In mathematical terms;
a^2 = b^2 + c^2
Let a represent the hypotenuse = 39 mm and the length of one leg, say c, is 15 mm
Slotting in the values of a and b
39^2 = b^2 + 15^2
1521 = b^2 + 225
collect like terms
1521 - 225 = b^2
1296 = b^2
Take the square root of both sides
36 = b
Therefore b = 36 mm