Please help, struggling and falling behind.

Suppose that in January, the maximum water depth of Lake Powell, AZ was 429 feet
and that the water evaporates at a rate of 2.9 feet per month.

a) Let y be the maximum depth of Lake Powell. Find an equation to model the depth t
months after January.

b) If it does not rain at all, what will the depth of the lake be in May?


c) If it does not rain at all, what will the depth of the lake be in July?

(Picture below)