Question 182861
4x^2 + 6.4x + k
.
Looking at just the terms with 'x':
4x^2 + 6.4x
Factor out 4:
4(x^2 + 1.8x)
.
Take 1/2 of 1.8 and square it, to get:
((1/2)(1.8))^2 = .9^2 = 0.81
.
Adding it in:
4(x^2 + 1.8x + 0.81)
4x^2 + 6.4x + 3.24
.
So, k = 3.24
.
The process above is called "completing the square".  For additional info see:
http://www.purplemath.com/modules/solvquad3.htm