Question 398078
If x gallons of a 14%-salt 
then obtain (x+6) gallons of a 17%-salt 
x gallons of a 14%-salt contains 0.14*x gallons of solt
6 gallons of a 21%-salt contains 0.21*6 gallons of solt
(x+6) gallons of a 17%-salt contains 0.17*(x+6) gallons of solt
So equation:
0.14x+0.21*6=0.17*(x+6)
0.14x+1.26=0.17x+1.02
0.03x=0.24
x=8 gallons of a 14%-salt