Marcario should use 4 pounds of macadamia nuts and 8 pounds of almonds.
Explanation
given
cost of macadamia nuts per pound=$9
cost of almond nuts per pound=$5.25
cost of the mixture per pound=$6.5
total pounds of the mixture to be made=12
let the pounds of macadamia nuts=x
pounds of almond nuts=y
total pounds of nuts=x+y=12
solving (1) and (2)
x=4
y=8