What factors might make it possible for a rogue wave to be more than twice the height of the two colliding waves?

Maybe if the tide was low, the water level would be lower, so that means the rest of the waves would be significantly smaller compared to the rogue wave.

Two big waves could reinforce each other to make one gigantic one through constructive inference.

Explanation from the Lesson:

complete the square on the right side of the equation;

-5(x + 0)^2 + 15

answer: d. y = -x^2 + 15