For the cheetah, note down what you have, and let t=t as you need to include it in your equation. When they start 10m apart, assume t=0. Write an equation for the cheetah's distance travelled.
Now for the impala. It travels at constant speed. Write down an equation for its distance but remember to add 10, as when t=0, its position is 10m.
The distance between them is (impala - cheetah).