Which of the following shows the extraneous solution to the logarithmic equation log7(3x^3+x)- log7(x)=2

To solve the give logarithm we proceed as follows;
log7(3x^3+x)-log7(x)=2
this can be written as:
log7[(3x^3+x)/(x)]=2

but;
log7(49)=2
thus;
log7(3x^3+x)/x=log7(49)
hence canceling log7 we get:
(3x^3+x)/x=49
This can be simplified further to get:
3x^2+1=49
3x^2=49-1
3x^2=48
x^2=48/3
x^2=16
getting the square root of both sides we get:
x=4
the answer is x=4