The daily mean temperature in a particular place is 83. how many cooling-degree days were accumulated?

Degree days are the difference between the daily temperature mean, (high temperature plus low temperature divided by two) and 65°F. 65°F because we assume that at this temperature we do not need cooling or heating. 
In our case the daily mean temperature is 83°F and X is the number of cooling-degree days.
X=83-65
X=18

X^2 - 6x + 2 = 0
Completing the square:-
(x - 3)^2 - 9 + 2 = 0

(x - 3)^2 = 9 - 2 = 7

x - 3 =  +/- √7

x  =   3 + √7 and 3 - √7     Answer