Question 173656
For a perfect square, the form is
{{{(x+B)^2}}}
{{{x^2 + 2Bx + B^2}}}

So let's get the original equation in the correct form
{{{4x^2 + 6.4x +k}}}
{{{x^2 + 6.4/4x + k/4}}}

Now use the values in your eqaution and set them equal to the ones that make up the 'correct form'
{{{6.4/4 = 2B}}}
{{{6.4/8 = B}}}


{{k/4 = B^2}}
{{{k = 4*B^2}}}
{{{k = 4 * (6.4/8)^2}}}
Now just grind that out and check your answer