In the problem, they gave you a force (F) and two acceleration values (you know it's acceleration because the units are m/s^2 while speed/velocity would be m/s).
We know that for an acceleration of 3.0m/s^2, we need to apply a force of 15N. We don't know the mass of the block however, and we need that to find the force necessary for an acceleration of 6.0m/s^2 (according to Newton's 2nd Law, F = ma).
So, let's apply F = ma to this problem to find the mass of the block!
Now we know the mass of the block thanks to the first givens, so we can figure out the new force!
Therefore, your answer should be 30N.