Given:
The Coffee Counter charges $7.00 per pound for Kenyan French Roast coffee and $11.00 per pound for Sumatran coffee
Let the number of pounds from Kenyan French Roast coffee = x
And the number of pounds from Sumatran coffee = y
He wants to make a 24-pound blend that sells for $9.00 per pound
So, we have the following equations:

From equation (1):

substitute with (x) from equation (3) into equation (2) then solve for (y)

substitute with (y) into equation (3) to find (x)

so, the answer will be:
The Coffee Counter should mix (12) pounds of Kenyan Roast coffee and (12) pounds of Sumatran coffee.