Let x represent the number of pounds of coffee that she bought.
From the information given,
cost per pound of coffee = $8.73
Cost of x pounds of coffee = 8.73 * x = 8.73x
Initial amount that she had = $90
After spencing $8.73x, the amount left is $63.81. This means that
90 - 8.73x = 63.81
8.73x = 90 - 63.81
8.73x = 26.19
x = 26.19/8.73
x = 3
She bought 3 pounds of coffee