You would start out by writing the two equations of the plans.
Plan 1: 170 + 8x
Plan 2: 93 + 15x
Then set both equations equal to each other:
170 + 8x= 93 + 15x
Then substrate 8x from both sides:
170=93+7x
Then subtract 93 from both sides:
77=7x
(Remember you can only subtract or add terms from/ to other like terms, ie: 7x-7x or 170-93 NOT 93-7x this would not work)
Then divide both sides by 7:
11=x
Your answer would be 11.