so check the picture below
if we check the solutions or zeros for the equation, we get [tex]\bf y=-x^2+6\implies 0=-x^2+6\implies x=\pm \sqrt{6}[/tex]
and as you can see in the picture, the value of "x" when y = 3.5, or the height of the truck is [tex]\bf y=-x^2+6\implies 3.5=-x^2+6\implies x=\pm \sqrt{2.5}[/tex]
the truck, ends up with a clearance of [tex]\bf \sqrt{2.5}-1.5[/tex] on either side, so, the maximum clearance, the truck can have, those two clearances added together, [tex]\bf 2(\sqrt{2.5}-1.5)[/tex]
