Log of a product is the sum of the logs of the multipliers: log7+log2x-log7-log(x+3) causes log7 to drop out so we get log2x=log(x+3). Therefore, 2x=x+3 and x=3.
Or we can combine all the logs under one log: log((7*2x)/(7(x+3)))=0. The number whose log is zero is 1 in any base, so 2x/(x+3)=1 and 2x=x+3 as before, making x=3.