Please help! Correct answer will get brainliest! How many gallons of a 21% acid solution and a 28% acid solution need to be mixed to make 7 gallons of a 25% acid solution???

Please help! Correct answer will get brainliest! How many gallons of a 21% acid solution and a 28% acid solution need to be mixed to make 7 gallons of a 25% acid solution???