3/6 because there are 6 sides and there are 3 numbers that you want to roll. They are even numbers so if you want to roll half of the numbers but not the other half well you have 3/6
Solve for c :
x⁴ + cx² + 100 = 0
cx² = -(x⁴ + 100)
c = - (x⁴ + 100)/x²
If x = 0, the right side is undefined and c would have no solution.
The answer is -51, hope this helps