Jennifer bought a textbook with 2/7 of her money. she then bought a notebook with 3/5 of her remaining money and was left with $12. how much money did she start with.

X=amount of money at first
price of a textbook=2/7 x
her remaining money after she bought his textbook was:  x  -2/7 x=5/7 x
price of the notebook: (3/5)(5/7 x)=3/7x
her remaining money after she bought his notebook was $12

We can suggest this equation:
2/7  x  +3/7 x + 12=x

lowest common multiple=7
2x+3x+84=7x
7x-2x-3x=84
2x=84
x=84/2
x=42

the amount of money at first was $42.