Find a value for k that will make 4x^2+6.4x+k a pefect square

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4(x^2 + 1.6x + .64)   factor out the 4, take half of 1.6 and square it to find .64 multiply the 4 back in to get 4x^2 + 6.4x + 2.56 which factors to (2x + 1.6)^2