Question

0.872g sample containing only NaBr and KBr gives 1.052g AgBr. what's the % of each salt?

This is the type of problem that cannot be answered without other information.

<b>I think you have made a typo in your problem I worked through it and I get negative numbers for the mass of NaBr and KBr. Of course that can not be. Perhaps the mass AgBr produced is 1.502 and not 1.052; that is, perhaps you transposed those numbers. Check it out.</b>
The problem is set up this way but you will need to substitute the correct values. I have used <b>1.502g for mass AgBr produced</b>.

You have two equations and two unknowns.
Let Y = grams KBr.
Let Z = grams NaBr.
Note: AgBr, KBr, and NaBr below stand for molar mass of those materials.
=======================
eqn 1 is Y + Z = 0.872
eqn 2 is Y(AgBr/KBr) + Z(AgBr/NaBr)=1.502

Solve for Y and Z which is grams of each.
Then determine % by
%KBr = (massKBr/1.502)*100 and
%NaBr=(mass NaBr/1.502)*100.

Post your work if you get stuck.