1/7+2x/3=(15x-3)/21 make all terms have a common denominator of 21
3+14x=15x-3 subtract 3 from both sides
14x=15x-6 subtract 15x from both sides
-x=-6 divide both sides by -1
x=6
*Notice the use of parentheses in this format to make it clear what the numerators and denominators are...
