A student solves the following problem:
Problem:
2(x-3) + 3x = 19
Step 1:
2x - 6 + 3x = 19
Step2:
(2x + 3x) -6 = 19
Step 3:
5х - 6 = 19
Step 4: 5x - 6 + 6 = 19 + 6
Step 5: 5x = 25
Step 6: x = 5
What property justifies going from step 3 to step 4?