I see you're probably trying to use a math editor to present this problem. Unfortunately, I'm unsure how to decipher your "-\frac {a }{8.06)." What fraction are you speaking of?
Given "#-\frac { a } { 8.06} + 7.02= 18.4#," all I can say with certainty is that you could legitimately subtract 7.02 from both sides:
#-\frac { a } { 8.06} + 7.02= 18.4#
- 7.02 = -7.02
After I've heard back from you, I'll try to help you solve the entire problem.