Need some help with some equations?

I'm reallllyy bad at math and I've attempted to do these but it just goes over my head, I've never been able to do math. Anyways, could anyone work out the following with steps so I can understand how to do it? Thanks!

A stone is thrown vertically upwards with a velocity of 20m s-1 is at a height y metres where y - 20t - 5t^2

For how long is it over 10m above the ground?

You want to find $y > 10 \Rightarrow 20t - 5t^2 > 10 \Rightarrow -10 + 20t - 5t^2 > 0$, sketch the quadratic $-10 + 20t - 5t^2$, find the roots, between which two values of $t$ is it greater than zero? The difference of those two t values if your length of time.

A batsman hits a cricket ball so that its trajectory has the equation y = x-1/80* x^2
where x and y are horizontal and vertical distances measured in metres. How far away does it land and what is its greatest height?.

Plot the graph of y = x-1/80x^2 - it's going to be a parabola/quadratic. It will start from the origin and the second root will be the distance it lands (imagine the graph being quite literally the path of the projectile), the maximum height will occur where? You tell me, what do you know about maximums/minimums of quadratic equations?

I know nothing, this is something we were "supposed" to be taught in GCSE, however with the fact that I was always in the lower set, they never bothered to teach us what we needed to know. Now I'm doing BTEC I'm struggling with the maths aspect of science because I never learnt what I need to know. Considering this is BTEC, minimal help is given.

A quadratic of the form $ax^2 + bx + c$ has a minimum/maximum point when $x = -\frac{b}{2a}$ and the value of that function at that point is given by just plugging $x = -\frac{b}{2a}$ into your quadratic.

ok o.O and what does the last equation that you put mean?

So you have some random quadratic like $1 + 2x + x^2$, the minimum/maximum value will occurs when $x = -\frac{2}{2 \times 1} = -1$, so the minimum/maximum value of the quadratic is $1 + 2(-1) + (-1)^2$ (i.e: you plug the value of x that gives the minimum into the quadratic to get the minimum value of the quadratic).

Oh I see