With that informatio you can:
1) Write the chemical equation
2) Balance the chemical equation
3) State the molar ratios
4) Predict if precipitation occurs.
I will do all four, for you:
1) Chemical equation:
mercury(I) nitrate potassium bromide mercury(I) bromide potassium nitrate
Hg2(NO3)2 + KBr → Hg2Br2 + KNO3
2) Balanced chemical equation
Hg2(NO3)2 + 2KBr → Hg2Br2 + 2KNO3
3) Molar ratios or proportions:
1 mol Hg2(NO3)2 : 2 mol KBr : 1 mol Hg2Br2 : 2 mol KNO3
4) Prediction of precipitation.
You can use the solubility rules or a table of solubilities. I found in a table of solutiblities that mercury(I) bromide is insoluble and potassium bromide is soluble, Then you can predict that the precipitation of mercury(I) bromide will occur.