They are both correct to an extent; however, Kole is more correct in the fact that he simplified the equation. Where Bella has the right idea but did not simplify the equation.
I agree with Kole because 220/11 =20 weeks until she has saved enough for her bike.
Both equations get you the correct answer though:
Kole: 265-45=220/11=20 weeks
Bella: 11w + 45 = 265 ... subtract 45 from both sides ... 11w = 220 ... divide both sides by 11 and you get 20 weeks