Conversion of transmitter power from Watts into dB
Thanks to @MBaz who pointed out my mistake: dBm and dBW are unitless, but they relate to  a reference of 1 mwatt, and 1 Watt. 

The question has an error, as well as the answer. Watts does not convert to dB, it converts to dBW. (i) should be as follows:

find (i) the transmit power in dBm and dBW. 

The reason behind that is that dB is a ratio, while dBm and dBW are ratios with a reference to 1 mWatt and 1 Watt, respectively, and you can use them to recover an absolute value of the power.

For example, the signal to noise ratio is a measure of how strong a signal is compared to the noise. In the linear scale it is written as:

$\frac{\text{Signal Power (in watts or miliwatts)}}{\text{Noise Power (in watts or miliwatts)}} = SNR (\text{unitless})$

or in logarithmic domain: 

$\text{Signal Power (in dBW or dBm)}-\text{Noise Power (in dBW or dBm)} = SNR (\text{in dB})$

So the correct answer to (i) is 

TX power = $10\log(50)=17\text{ dBW}=17+30=47\text{ dBm}$