Not exactly sure what the question wants either, especially since both solutions are equally valid. I would simply substitute the solutions as two separate scenarios to show that they are both valid.

Scenario 1: x = 70 | Mindy bought 70 books

Cost price per book: $168 ÷ 70 = $2.40
# of books left after selling 20 books: 70 – 20 = 50
Price which the remaining 50 books were sold: $2.40 – $2 = $0.40
Earnings from selling these 50 books: 50 x $0.40 = $20

Profit: ($240 + $20) – $168 = $92

Scenario 2: x = 24 | Mindy bought 24 books

Cost price per book: $168 ÷ 24 = $7
# of books left after selling 20 books: 24 – 20 = 4
Price which the remaining 4 books were sold: $7 – $2 = $5
Earnings from selling these 4 books = $20

Profit: ($240 + $20) – $168 = $92

Since both solutions are positive and real roots which satisfy the equation, both are equally valid in this context.