If 25x^4-20x^3+14x^2+hx+k is a perfect square, then the square root must be of the form (5x^2+ax+b), where a and b are constants.
(5x^2+ax+b)^2=25x^4+10ax^3+(a^2+10b)x^2+2abx+b^2. Comparing coefficients we see:
x^3: 10a=-20, so a=-2
x^2: a^2+10b=14, 4+10b=14, so b=1
x: 2ab=h=-4
constant: b^2=k=1