You know the molar concentration, 2.9 x 10^-6 M, which means you have 2.9 x 10^-6 moles in one liter of solution. One liter is 1000 mL or 1000 g since the density of water is 1g/ml
1. Find the formula mass of Cd(NO3)2
2. Multiply the formula mass * the molarity (2.9 x 10^-6 M), this tells you how many grams in 1000 grams. Let this be X grams.
3. Set up a ratio: X grams (from #2)/1000 g = Y grams/1000000 g
Solve for Y, the # of grams in one million grams of water, which is also the ppm.