-1+2p=-10+3p
if you add or subtract the same thing to both sides, the statement is still true.
eg. 5=5. if i add 1 to both sides, 5+1=5+1 which is 6=6 which is still true.
using this, if i add 10 to both sides, it becomes
9+2p = 3p as it has cancelled out all of the 'normal' numbers from the right.
next, if you subtract 2p from both sides, it will take all the p's off the left, giving
9=p
which is the answer for p :)