Answer:
b = 2(g + r)
Marisa used b = 8 cups of blueberries.
Explanation:
Let g be the cups of green grapes, r be the cups of red grapes, and b be the cups o blueberries Marisa used.. 
then we know that 
g = 1 (Marisa used 1 cup of green grapes)
r = 3 (Marisa used 3 cup of red grapes)
g + r = 4
Marisa used twice as may cups of blueberries as cups of grapes; this means 
b = 2(g + r ) 
b = 2(4) = 8
Marisa used 8 cups pf blueberries.