7-4x=21+3x

7 - 4x  =  21 + 3x

Get all the x-terms to one side by getting rid of the maller x-term:  add  4x  to both sides:

7 - 4x + 4x  =  21 + 3x + 4x

7  =  21 + 7x

Get the x-term alone on its side:  subtract 21 from both sides:

7 - 21  =  21 - 21 + 7x

-14  =  7x

Divide both terms by 7:

-2  =  x