Answer
7.9 ppm
Work Step by Step
There are 0.0079g of Sr2+ per kilogram of water. 1kg is 1000 grams so we have
(0.0079g Sr 2+ / 1000g water)
ppm is parts per million or parts per 10^6 so we set up a ratio:
(0.0079g Sr 2+ / 1000g water) = x / 1 x 10^6 g water
and solve for x, x = 7.9 g or 7.9ppm