Q:Related rates problem
Let x be 1.
The kayaking rate is unaffected by the swim speed, only the current, so you should solve for the current, z, with Equations 1 and 3.
As a hint, don't do 45(y-z)=40(y+z), as that will give you z in terms of y when you can solve for z itself.
Assuming s is the kayaking time from A to B, notice equations 2 and 4 are two equations with 2 unknowns after you find z, namely a and s.
Can you take it from here?