She needs a total of $750, so set m to 750 and solve for p:
M = 0.18p +147
m = 750:
750 = 0.18p + 147
Subtract 147 from each side:
603 = 0.18p
Divide both sides by 0.18:
p = 603 / 0.18
p = 3350
She will need to deliver 3,350 phone books.
