Oh sorry! I thought the particle was projected horizontally lol my bad.
i think the question is saying that at x=30 and y=26.25, it is the maximum height, so...
y=-1/2gt^2 + Vtsin@ + 15, y'=-gt+Vsin@, when y'=0, t=Vsin@/g,
Sub t into y: y=......(sorry getting lazy)
Then you put...