Aquestion of life or death !

find the value of b that makes x^2+bx+81 a perfect square trinomial.

To be a perfect square this condition must be applied b^2 -4ac>=0
where a is coefficient of x^2 and b is coefficient of x and c is the last term
so in this equation
a=1 ; b = ? ; c= 81
b^2 -4ac >=0
b^2 -4(1)(81)>=0
b^2 >= 324
so b= 18

The answer would be (c)

B. 16is the absolute value