The quadratic equation whose roots are equal, then

If $b^2-4ac=0(\Delta =0)$,then the roots are real and equal

Example: $x^2-8x+16=0$

$a=1,b=-8,c=16$

$\Delta =b^2-4ac$

$={(-8)}^2-4\left(1\right)\left(16\right)$

$=64-64$

$=0$