<span>Twice the sum of x = 2x
2x </span>≤ 38
x ≤ 38 ÷ 2
x ≤ 19
Greatest value of x is 19
First off, let's change the format of the percentage to decimal.. so 35% is just 35/100 or 0.35 and 40% is 40/100 or 0.4 and so on.

now, whatever "x" amount is, we know that x + 10 must add up to "y". x + 10 = y, because both quantities added will give the mixture amount.
and whatever 0.35x + 4 = 0.37y, because both concentrated amounts must give the 37% of the mixture.

solve for "x", to see how much of the 35% solution will be needed.
what about "y"? well, x + 10 = y.
X-intercepts are found by factoring. Use the quadratic formula on the first since it's in standard form and you find that your x values are in fact -3 and 7. For the second one, use the Zero Product Property that says that x - 3 = 0 or x + 7 = 0. Therefore, x = 3 and -7. Signs are wrong. So not the second one. As for the third one, if you factor out a 3, your polynomial is exactly the same as the first one which did give us the desired x values. So the third one checks out. If you FOIL out the first one and then apply the quadratic formula you do get x = 3 and -7. So the fourth one checks out too. For the last one, putting it into the quadratic formula gives you x values of 3 and -7, so no to that one. Summary: 1st, 3rd, 4th have zeros of -3 and 7; 2nd and 5th do not.