Step-by-step explanation:
If 1 gallon = 3.785 litres
Then, 10 gallons = 37.85 litres
Given

To obtain the minimum value of y, we first take the derivative of y
The derivative of y is:

Equating

gives the minimum value we require.
Doing that, we have:

So that

Therefore, the minimum value is x = 3
2(b+3) = 2
2b + 6 = 2 ( -6 )
2b = -4 ( / 2)
b = -2