Which of the following must be true for and expression to be a difference of two squares?

a. both coefficients are perfect squares.

b. there are only two terms.

c. both terms have negative coefficients.

Let's say e=a^2-b^2 and take each affirmation:

a. both coefficients are perfect squares.

a could be equal to 2 which is not a perfect square

b. there are only two terms - obviously yes

c. both terms have negative coefficients - as we can see just b has a negative coefficient

Final answer b. there are only two terms.