You will need to work left to right.
p - 10p + -5p +14p - -9p - 2 = -20
-9p + -5p +14p - -9p - 2 = -20
-14p +14p - -9p - 2 = -20
0p- -9p - 2 = -20
9p - 2 = -20
Then you will want to eliminate 2 by adding it to both sides
9p = -18
Then divide by 9 to isolate P
p=-2