Q. Two clay pidgeons are launched vertically upwards from exactly the same spot at 1 s intervals. Each clay pidgeon has an initial speed 30 m/s and acceleration 10 m/s downwards. How high above the ground do they collide?

I know i need to find out when both the pidgeons have the same height using a suvat, but im not sure how i would use an equation as i only have the acceleration and the initial speed.

Can anyone give me a hint, or even tell me the whole method. Thanks in advanced.