Solve for x (3x + 1)/7 - (3x + 5)/3 = (x+3)/21.

Before computing x, we'll have to calculate the least
common denominator of the 3 ratios.

LCD =
3*7

LCD = 21

Now, we'll
multiply the first ratio by 3 and the second ratio by 7. The 3rd ratio has the
denominator 21, so it won't be multiplied.

We'll re-write
the equation, all 3 quotients having the denominator
21;

3(3x + 1) - 7(3x + 5) =
(x+3)

We'll remove the
brackets:

9x + 3 - 21x - 35 = x +
3

We'll move all terms to one
side:

9x + 3 - 21x - 35 - x - 3 =
0

We'll combine and eliminate like
terms:

-13x = 0

We'll divide
by -13:

x =
0

The solution of the equation is x =
0.