i believe you are referring to suvat equations, those can work, but i never remember them. let's start from scratch

the acceleration due to gravity is -9.81 m/s

thus we have:

with the initial condition, m/s and

now, let's find and

(by the way is the position function, is the velocity function and is the acceleration function)

so,

since , we have:

thus,

we obtain the max height when

that is, when

plug that into the position function:

since , we have that here.

since we obtain the max height at , the max height is,

i think you can handle the rest

...by the way? have you done calculus? if not, the suvat equations are the way to go, forget everything i said and see here (note that these equations assume all initials are zero.