I've had plenty of students who could recite F=ma and could readily solve for F, given m and a. But, given F and a, they lacked the understanding of how to solve for m.
This little trick solved a lot of problems (and even students capable of solving for m enjoyed using this).
Draw a triangle and divide the triangle into three parts by drawing a T in it (see below).
Now fill in the variables. In the case of F=ma, the F goes on the top and m and a each go in a bottom section.
Use your finger to cover the variable you're solving for an "read" off the equation.
If you're solving for F, cover the F and you'll notice the m and a are next to each other, which means they need to be multiplied to get F.
If you're solving for m, cover the m and you'll notice that you're left with F over a, so you'll need to divide F by a get get m.
And finally, if you're solving for a, cover the a and you'll notice that you're left with F over m, so you'll need to divide F by m to get a.
Much like the popsicle stick, this trick can work for any three variable equation like density and speed.
As long as you can remember one iteration of the formula, you can recreate the triangle!