Answer
a. 3
b. 5
c. 4
Work Step by Step
a. Axiom 3, addition is associative
$(u+v)+w=u+(v+w)$
b. Axiom 5, the existence of an additive inverse
For $u$ there exists $-u$ such that $-u+u=0.$
c. Axiom 4, the neutral (zero) element for addition