To get you started, let x = the number of gallons of 90% antifreeze
0.90x + .20(6060) = .80(x + 6060)
Note that: 90% = 0.90 , 20% = 0.20 , 80% = 0.80
The left hand-side of the equation represents the concentration and individual volumes of each antifreeze solution
The right-hand side of the equation is the resulting sum and concentration of the mixture
To solve the equation for x, you can multiply both sides of the equation by 100 to clear the decimals.
You should be able to handle the rest, Derrick.