How do i find the critical points for an analytic graph

The depth of a lake at a certain point, wich is a function of the distance of that point from shore, is 30 feet.

Find the y-intercept and x-intercept for the equation 5x-3y=60 what is the y-intercept

The nth term of a sequence in 2n-1 the nth term of a different sequence is 3n+1 work out 3 numbers that are in bothe sequences and are between 20 and 40