Consider the quadratic equation given by:
$$ ax^2 + bx + c = 0 $$
where $ a, b, c $ are constants. The quadratic equation has roots that are equal, meaning the discriminant must be zero.
Suppose $ a = 2 $, $ b = 8 $, and $ c = k $. Find the value of $ k $ such that this equation has equal roots, and enter the value in the grid below.