I am using the equation F=ma (force equals mass times acceleration) to solve these problems.
1. You are looking for force, and have mass and acceleration. You just plug in the values for mass and acceleration to get the force needed.
F=(15kg)(5m/s^2)
F=75N
2. Again, you are looking for force, and just need to plug in the values for mass and acceleration
F=(3kg)(2.4m/s^2)
F=7.2N
3. In this problem, you have force and mass, but need to find acceleration. To do this, you need to get acceleration alone on one side of the equation - divide each side by m. Your equation will now be F/m=a
a=(5N)/(3.7kg)
a=18.5m/s^2
I did not use significant figures. Let me know if you need to do that and need any help on that. Hope this helps!