Answer:
By whether x^2 has a negative or positive coefficient.
If the parabola opens down, x^2 has a negative sign in front of it.
If the parabola opens up, x^2 does not have a negative.
Explanation:
Completely expand a quadratic equation into this polynomial form to find “a”:
ax^2 + bx + c
When “a” is positive, the parabola opens up.
When “a” is negative, the parabola opens down.
