The length of the hypotenuse of a right triangle is 145 units. The length of one leg of the triangle is
144 units. Mike wrote the following step to find the length of the unknown leg:

Length of the unknown leg = 1452 − 1442 = 21,025 − 20,736 = 289 units

Which statement best explains whether Mike's step is correct or incorrect?

It is incorrect because the length of the unknown side is the square root of 289.
It is incorrect because the length of the unknown side is the square root of 41,761.
It is correct because the length of the unknown side is the difference of the lengths of the sides.
It is correct because the length of the unknown side is the difference of the squares of the sides.