Answer:
She bought 5 pounds of coffee.
Explanation:
We are looking for how many pounds so pounds would be your x. 
each pound costs $8.62. 
You are taking the money she payed for the coffee away from the $70 she received. 
Set up your equation 
70 - 8.62x = 26.90
Solve for x 
Subtract 70 from both sides 70-70 (Cancels out) - 8.62x= 26.90-70
-8.62x = -43.10
Divide -8.62 from each side. -8.62/-8.62(cancels out) x = -43.10/-8.62 (two negative numbers multiplied or divided by each other makes a positive)
x=5