Help with geometric proofs

Prove Theorem 2.6- angles supplementary to same angles are congruent.
Given: angle 1 and angle 2 are supplementary; angle 1 and angle 3 are supplementary.
Prove: angle 2 is congruent to angle 3.
Thanks in advance! :D

L1 and L2 are supplementary - given
L1 and L 3 are supplementary - given
L1 + L2 = 180 - definition of supplementary
L1 + L3 = 180 - definition of supplementary
L1 + L2 = L1 + L3 - substitution
L2 = L3 - subtraction
L2 congruent L3 - definition of congruence