You should start with an equation.

The 'x' represents the week.
so now, we have to subtract 125 from the total amount which is 245.

now we have

to find 'x', we must divide 15 on each side

we are now left with

that means Sierra will have 8 weeks until she has enough money to buy the bike
The coefficient of the second term is 18.
What you do is you FOIL the terms: