So, let's build up a simple equation to solve that!
I have $90 total. After x pounds of coffee that costs $7.82 each, I'm left with $43.08. So,
90 - 7.82x = 43.08 (Let's subtract 90 in both sides)
-7.82x = 43.08 - 90
-7.82x = - 46.92 (Multiply by -1)
7.82x = 46.92 (divide by 7.82 on both sides)
x = 6 pounds
She bought 6 pounds of coffee.