Hello Stephen,
I think you might be missing numbers for i0 which typically equals 10-12.
You know L = 150 dB and i0, so you're solving for i:
150 = 10 log(i/10-12).
Divide both sides by 10: 15 = log(i/10-12)
From there: you can convert this log equation into an exponential equation: 1015 = i/10-12
Multiply both sides by 10-12: 1015 x 10-12 = i; therefore i = 103 = 1000
Answer: i = 1000