28 because a constant is a number that doesn't have x attached to it.hope this helps
So check the picture below
if we check the solutions or zeros for the equation, we get

and as you can see in the picture, the value of "x" when y = 3.5, or the height of the truck is

the truck, ends up with a clearance of

on either side, so, the maximum clearance, the truck can have, those two clearances added together,