For part (a) think of the horizontal changes as changes in x and the vertical changes as y in a right triangle XYZ.

differentiate...

Divide by two to ease up on the numbers

You know that

because the ladder is 7 meters from the base of the wall. You know that

because the ladder is decreasing the gap between the wall and the ladder at 2 meters per sec. You know that

because of the Pythagorean theorem. Moreover you know that

because z always equals 10.

Plug in those numbers and you will get

From that, you get

meters per second.