Help urgent maths

You need to think of overall positive direction, rather than comparing to the height. If you're treating downward height as negative, then "up" is your overall positive direction, so if gravity is acting down then it's also negative.

(The mark scheme is treating "down" as positive, but the same logic applies.)

So is height and gravity usually the same sign (positive or negative)???

No, in this case they are because the particle has overall moved in the same direction as gravity is acting, but that need not be the case.

If it helps, when you get a question like this, draw a picture, and draw an arrow to the side of it to be your positive direction. Then draw arrows for each force and movement (e.g. height in this case): if an arrow is in the same direction as your original arrow, then that value is positive, and if not then it's negative.

e.g. say we designate up to be positive (^). The particle is going down (v), so that arrow points in the opposite direction, so height is negative (as you found). Gravity is acting downwards (v), so again that's the opposite direction to our positive arrow, so it's also negative.

(So for the mark scheme, they've designated down to be positive (v). Since our height and gravity arrows are both also down (v), we conclude that they're both positive. It doesn't matter which direction you choose to be positive, as long as you're consistent throughout your calculation, as tonyiptony said.)

Ahhh I see thanks..
But how come when we take down to be negative…
Gravity is also negative… because the force gravity acts down always (so wouldn’t it be positive) do you get what I mean?? I’m so stuck