a.  The power levels in dBm are determined by substituting into Equation 3:
dBm = 10 log(\frac{P}{1\,mW})          (3)
dBm = 10 log(\frac{10\,mW}{1\,mW}) = 10 dBm
dBm = 10 log(\frac{0.5\,mW}{1\,mW}) = -3 dBm

b.  The difference between the two power levels in dB is determined by substituting into Equation 2:
dB = 10 log(\frac{P_{1}}{P_{2}})          (2)
dB = 10 log(\frac{10\,mW}{0.5\,mW}) = 13 dB
or        10 dBm – (-3 dBm) = 13 dB
The 10-mW power level is 13 dB higher than a 0.5-mW power level.