subtract 3 from both sides
simplify 12 - 3 to 9
break down the problem into these two equations
1 + p = 9 and -(1 + p) = 9
solve the first equation 1 + p = 9 and that would be 8 since 1 + 8 = 9 is true.
solve the second equation -(1 + p) = 9 and just simplify brackets and add 1 to both sides then add 9 + 1 and lastly multiply both sides by -1 and p = -10.
Gather both solutions
Answers: p = -10, 8
