Let c = the number of city miles George gets per gallon. Then if he drives 150 miles in the city, his car burns 150/c gallons doing so.
Now let h = the number of highway miles George gets per gallon. If he drives 112 miles on the highway, he uses 112/h gallons doing so.
But you are told that h = c + 3, so let's substitute "c + 3" in place of the "h". So George's car burns 112/(3 + c) gallons on the highway.
Since George used up 10 gallons in all for his trip, then you know that
150/c + 112/(3+c) = 10.
Now you have an equation with just one variable in it. Re-arrange it to solve for c - that's the answer you are seeking.
Hope this helps,
Stephen La Rocque>