and also how to determine the range of it.
The general answer is too long to post. Post an example.
Write the equation in vertex form.
y = a (x - b)^2 + c
If a is negative then it opens down, otherwise it opens up.
The vertex will be at (b, c)
If it opens up then c is the lowest value in the range. If it opens down then c is the highest value in the range. The range will be either [c, ?) or (?, c] depending on which way it opens.