You are solving for x.
Isolate the x. Note the equal sign. What you do to one side, you do to the other.
5x - 7 = 21
5x - 7 (+7) = 21 (+7)
5x = 28
5x/5 = 28/5
x = 28/5
x = 5.6
C) should be your answer, because it is the one that meets the requirement of placing the dot on 5.6 (or approximately)
hope this helps