There is nothing added to or subtracted from the term involving the " r ".
So we can simply put the " p " and the " t " underneath both sides of the
equation and then cancel them away on the right, leaving only the " r ".

So we put " p*t " underneath both sides of the equation, like this



Then we cancel the " p "'s and the " t "'s on the right, like this:



Then all that's left on the right that didn't cancel was the " r ".  So we
have:



And then it's customary to write the final answer so that the letter
being solved for is on the left. 



Edwin