If the zeroes of the quadratic polynomial $a{x}^2+bx+c$, c≠0 are equal, then 

For equal roots, the discriminant will be equal to zero. 

$b^2 -4ac = 0$ 

$b^2 = 4ac$ 

$ac = b^2/4$ 

ac>0 (as square of any number cannot be negative)