A)
Let s and g be the number of steel and graphite sets sold...then:
s+g=11 and
309s+489g=4299
B)
solving the first for g, g=11-s, then substituting this value of g in the second equation gives you:
309s+489(11-s)=4299
309s+5379-489s=4299
-180s+5379=4299
-180s=-1080
s=6, and since s+g+11
g=5
So the store sold 6 steel sets and 5 graphite sets.