I won’t do your homework.

I will give you a page that will help.

Here is an example of how to calculate atomic weight.

What is different in your problem is that the relative amounts are given as a ratio, 2:25. To convert this to fractional abundance, we note that of 27 units of the Lithium,
2 will be Lithium-6 and 25 will be Lithium-7, so the respective fractions are 2/27 and 25/27. That should allow you to solve the problem.