E=cm/r
To solve for m, you have to get it by itself on one side of the equal sign.
To do that you have to eliminate c and r and put them at the other side.
r is being divided by m, so you have to do the opposite to get it on the other side, which is multiply. Multiplying r to both sides will remove the division, and give you Er=cm. So... B is the answer :)
