Which solution shown below contains an error?

What is the missing reason in step 5? linear pair postulategivendefinition of complementary anglescongruent complements theorem

Kelly is a salesperson at a shoe store, where she must sell a pre-set number of pairs of shoes each month. at the end of each work day the number of pairs of shoes that she has left to sell that month is given by the equation s=300-15x , where s is the number of pair of shoes kelly still needs to sell and x is the number of days she has worked that month. what is the meaning of the number 300 in this equation

Iwill give you ! what is the measure of ∠v ?