There are four possibilities:

It snows in neither city

P1 = (0.6)(0.3)

It snows in both cities

P2 = (0.4)(0.7)

It snows in the first city only

P3 = (0.4)(0.3)

It snows in the second city only

P4 = (0.6)(0.7)

The probability that it snows in exactly one city is simply P3 + P4.