First let us see what in formation we have:
s - Displacement - 50 metres
u - Initial velocity - 30m/s
v - Final velocity - N/A
a - Acceleration - 9.8m/s
t - Time - ?
We will take all values as downward, that is why we have acceleration as a positive value.
From the above information you should notice that we can use the following formula
Subbing in our values gives us,
Which is just a normal quadratic.
Hope this helps