Find a way to make one of the unknowns equal. since we are trying to work out the price of a pound of walnuts, I am going to make a equal on each side to leave us with w. This leaves us with:
15w+12a=165 6w+12a=102
Then take them away to leave w:
9w=163
Then solve:
w=163/9 = 18.11111111...
Then round it to 2 decimal places as the measurement is currency.