Three roots of the polynomial equation x4- 4x3 - 2x2 + 12x + 9 = 0 are 3. -1 and -1. Explain why the fourth root must be a real number. Find the fourth root.

Can someone explain this in a simple way??

from the figure we get,

the x axis shows   the months and,

the y-axis shows the number of satisfied customer.

each line or each cell in upward direction (y-axis) shows 500.

the appropriate scale for the y - axis is: -

a = 2000 customers

b = 4000 customers

c = 6000 customers

d = 8000 customers

e = 10000 customers

f = 12000 customers

g = 14000 customers

you need to add a picture