What is the reason for step 2 in this proof?

given:

angles a and b are supplementary.
the measure of angle a = 45º.

find the measure of angle b. proof:

1. angles a and b are supplementary and angle a = 45º. (reason: given)

2. measure of angle a + measure of angle b = 180º ( reason: ? ? )

3. 45º + measure of angle b = 180º (reason: substitution)

4. measure of angle b = 135º (reason: subtraction property of equality)

a) substitution property of equality
b) definition of supplementary angles
c) transitive property of equality