Question 743366
Learn the distinction between "gas" and "gasoline".  A reader could easily take your question to mean 6 gallons of something in the gaseous state of matter.  The question may, in fact, mean that, but seems unclear.  


 But, to the solution.


VARIABLES
T = 88% wanted
L = 87% source
H = 93% source
M = 6 gallons, mixture wanted
u = how much of Low percent gas
v = how much of High percent gas


FORM EQUATIONS
{{{(Lu+Hv)/M=T}}} and {{{u+v=M}}}


SOLUTION METHOD
Solve the system symbolically for u and v.  Substitute given values to evaluate u and v.