Solutions of mercury (i) nitrate and potassium bromide are mixed

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.