An object is traveling at a constant velocity of 15m/s when it experiences a constant acceleration of 3.5 m/s2 for a time of 11's. what will its velocity after that acceleration?

That all depends on the direction of the 15 m/s and the direction of the acceleration. You've only given us a speed and a magnitude of acceleration, with no directions. So we have no initial velocity, and no way to calculate a final one. / / / IF everything is in the same direction, then the final speed in that same direction is (15 m/s) + (3.5 m/s^2 x 11 sec). That's (15 m/s) + (38.5 m/s) = 53.5 m/s.

The independent variable would be the fertilizer as it is what the dependent variable (the growth of grass) depends on.