Tim is k years old and Jennifer is 4 years younger than Tim. Find their average age in 3k years' time, in terms of k.

Write both Tim and Jennifer's age in terms of k

Tim = k

Jennifer = k - 4

Now in 3k years

Tim = k + 3k

Jennifer = k - 4 + 3k

Simplify

Tim = 4k

Jennifer = 4k-4

Now to find average

(Tim + Jennifer) / 2

Plug in

(4k + 4k-4) / 2

(8k-4) / 2

Simplify

4k-2

I hope this is what you're looking for!