p - (-3) would turn into p+3.
So we have
p - (-3) = 2
become
p + 3 = 2
To isolate p, we need to move that "+3" over. To do this, we subtract 3 from both sides. Subtraction is the opposite (or inverse) of addition, which is why this works
p+3 = 2
p+3-3 = 2-3 ... subtract 3 from both sides
p+0 = -1
p = -1
So the final answer is p = -1