When it's folded up the height is x, the width is 70-2x and the length is 100-2x. We multiply them together to get the volume.
[tex]V = x(70-2x)(110-2x)[/tex]
That's part a, what you have. I'm not going to obsess about the units.
Part b, state which xs give a valid V
None of the factors should be negative. So that means x is nonnegative. I'm explicitly allowing the zero height, zero volume case as a case where the expression is valid. We'll get one negative factor again when 70<2x or 35<x so the range that gives all non-negative factors is
[tex]0 \le x \le 35 \textrm{ cm}
[/tex]
Part c, sketch the graph. I'll pass that one to Wolfram Alpha.
