To do this, the trinomial must be a perfect square trinomial. This means that the binomial must repeat itself exactly to FOIL out to the trinomial.
(5x+ )(5x+ )
The blanks must multiply to equal k and add together to equal 40 when multiplied by 5x.
(5x+4)(5x+4) = (5x+4)^2
Applying the FOIL method to the perfect square binomial shows that k=16