In part (a), you need to divide the volume of the cuboid by its base area. Find the base area by taking the square of the length, which you will expand accordingly and the result will be in surd form. When dividing a surd expression by another surd expression, the denominator has to be converted into a non-surd form. Recall that to do this, you make the denominator into the form (a + b) (a - b) so that you can expand it into a^2 - b^2. The denominator just happens to be 1, which leaves the final surd expression as shown.

In part (b), recall that in a quadratic equation ax^2 + bx + c = 0, surd roots will always be in conjugate form. Using the equation to find the conjugate roots of the equation, you can prove that the roots found cannot be conjugates of each other, thus proving the answer wrong.