Broken down into steps:
1. Find the slope of the line segment that connecs the points (0,-7) and (4,-15).
2. Start with the point-slope formula for the equation of a straight line:
y-b = m(x-a), where the given point is (a,b). Borrow the value of m that you calculated in (1), above, and insert it into this point-slope formula.
Finish up by subst. the x- and y-values in (-3,6) into this formula.
Done!
You could, of course, solve this result for y if you wished.