Question 382688
The polynomial becomes

{{{(x-1)(x-2)(x-3)(x-r) = 0}}}, where r is the fourth root. By Vieta's formulas, the sum of all roots of a polynomial {{{ax^n + bx^(n-1) + ... = 0}}} is -b/a. Since the {{{x^3}}} coefficient is zero, the sum of all four roots is zero, so it follows that r = -6.

The polynomial is now {{{(x-1)(x-2)(x-3)(x+6) = 0}}}. The constant term c is just (-1)(-2)(-3)(6) = -36.