Start with the information we're given or what we know:
Q = 5P (The measure of angle Q is five times the measure of angle P)
L = 2P + 12 (The measure of angle L is twelve more than twice the measure of angle P)
Q + P + L = 180 (We know that the angles of a triangle add up to 180°)
Let's substitute what we know to get P as the only variable in order to solve for it:
Q + P + L = 180
(5P) + (P) + (2P + 12)
5P + P + 2P + 12 = 180
8P + 12 = 180 (Combine like terms and solve for P)
-12 -12
8P = 168
8 8
P = 21
We can check our answer:
If Q is five times P then Q = 5P = 5(21) = 105
If L is 12 more than twice P then L = 2P + 12 = 2(21) + 12 = 42 + 12 = 54
105 + 54 + 21 should equal 180 and it does so our answer is correct
The measure of angle P is 21°