Let's say Lily invested x amount of money in bonds and 2x in stocks. What the yield is annually from those is 6% of x plus 2 * 8% of x, as she gets 6% in bonds and 8% in stocks. Simplified is 0.06x + 0.16x. Add and you get 0.22x. Now you can solve a simple equation.
Equation:
0.22x = 325.6 (as annual income is $325.60)
----divide both sides by 0.22----
x = 1480
----multiply by 2 to get your answer----
2x = amount of stocks = 1480 * 2 = $2960