Sketch the triangle and label the known information.
The question is asking about the rate-of-change of area.
Notice that there are two variables. It would be easier to have only one.
Use the Pythagorean Theorem to find the length of the height in terms of b.
Then rewrite the function with only one variable:
the rate that the area is changing.
Substitute in all values that you know, including the specific base length, 5.