The mass is irrelevant, because inertial and gravitational mass are equivalent.
One equation is s=ut-½gt² , where u=15.6m/s, g=9.8m/s², t is time, s is vertical distance.
So s=15.6t-4.9t². This is a parabola, an upturned U, so its vertex is a maximum—maximum height.
This can be written s= -4.9(t²-(15.6/4.9)t).
We can complete the square:
s=-4.9(t²-(15.6/4.9)t+(7.8/4.9)²-(7.8/4.9)²),
s=-4.9((t-7.8/4.9)²-(7.8/4.9)²,
s=-4.9(t-7.8/4.9)² +7.8²/4.9.
The maximum height occurs when the parenthesised expression is zero that is when s=7.8²/4.9=12.4m approx.