Where did i go wrong?

https://3b0a7b1bc87f5381e60f8f717510b7e3072e9617.googledrive.com/host/0B1ZiqBksUHNYcEVTUFdwYmtsb2c/June%202013%20(R)%20QP%20-%20M1%20Edexcel.pdf

question 4

i got
s=2t-4.9t²-50
and
s=20-4.9t²

however when i equated them to find time i got a negative, if this was a real exam i'd obviously disregard the negative and just make the answer positive xD since it's time

18t=-50

I believe you went wrong in your first suvat, it should be $s=2t-4.9t^{2}+50$ as when t=0, the ball is 50m above the ground.

but i said s+50 not s-50

for the first particle it is 50 m above ground, thus taking ground height as S the first particle must have height s+50 not s-50 xD

The reason it that suvats lie to you. It is formally $s=s_{0}+ut+\frac{1}{2}at^{2}$ where $s_{0}$ is the initial displacement of the particle. In this case $s_{0}=50$ so it is $s=50+2t-4.9t^{2}$

The problem your equation is that when $t=0$, $s=-50$