5/7x+1/7=3
multiple both sides by 7
7•5/7x+7•1/7=7•3
reduce the numbers with 7
5x+7•1/7=7•3
5x+1=7•3
5x+1=21
move the constant to the right side of the equation and change its sign
5x+1=21
-1 -1
5x=21-1
subject the numbers
5x=20
divide both sides of the equation by 5
5x=20
÷5 ÷5
x=4