设直线x-3y+4=0的倾斜角是a,则有tana=1/3
直线L的倾斜角是2a,则tan(2a)=2tana/(1-(tana)^2)=(2/3)/(1-1/9)=3/4
即直线的斜率k=tan2a=3/4
故方程是:y-1=3/4(x-2)
即:y=3/4x-1/2.