The answer will be 7xy⁻². 

Explanation:

1) The letter 'x' and 'y' are variables. Variable can vary from value to value.

2) When there are variables multiplying or dividing, you can treat them as an object. For example think of 2h as 2 apples. The term 'h' is replaced by apple(s).

Similarly, we can write 2cb as 2 oranges, here the term 'cb' is replaced by orange(s).

3) All the numbers and variables have some power, even when it is not written. For example: c is the same as c¹ or 7 is the same as 7¹. So it does not matter if a variable has a negative power, we will treat it the same as if it had a positive power.

Now think of xy⁻² as any object say table. Then 5 tabes + 2 tables are 7 tables.

tables = xy⁻², therefore 5xy⁻² + 2xy⁻² = 7xy⁻².

Hope this explains your doubts.

Extra information:

g⁻ᵇ = 1/gᵇ

So y⁻² = y² (1 upon y squared).

5⁻² = 1/25 = 0.04.