Can you please explain? Or maybe continue your question please?
First, recall that

So if
, then
.
Second,

We know that
and
, which means we should also have
.
Third,

but as we've already shown, we need to have
, so we pick the negative root.
Finally,

Unfortunately, none of the given answers match, so perhaps I've misunderstood one of the given conditions... In any case, this answer should tell you everything you need to know to find the right solution from the given options.
Multiply number of gallons by 16 (because 16 oz/ 1 gallon).
124=100+20+4, 17=10+7
124*17=(100+20+4)(10+7)=1000+200+40+700+140+28=2108