Finding the power output when one single sideband at m=0.4 is transmitted together with the carrier component which is reduced by 26dB
You have a 100% modulated carrier producing a power of 24kW. You need to calculate that part of the power that is only the carrier. Without carrier power you cannot calculate the sideband power for different modulation indexes.

From memory (and double checked on google), the carrier power m=1 (100%) is two-thirds the total power.

Therefore, basic carrier power is 16 kW. 

Now, what would be the sideband power when m=0.4?

Power in an AM signal is \$P_{carrier}\times ( 1 + 0.5*m^2 ) \$ and m = 0.4 therefore total power is 1.08 x 16kW = 17.28 kW. 

Subtract the 16kW carrier power leaving 1.28kw (this is the power in both sidebands). 

Divide that by two to get a single sideband power =0.64 kW

Then reduce 16kW by 26dB to get 40.12 watts.

Then add it to the single sideband power to get 0.6802kW.

Here's the link to the google book where I refreshed my memory