Question

3x+7=5x-21

Answer 1

Just get only one of  the placeholder on one side by itself
and do the same thing to both sides so it stays equal
3x+7=5x-21
add 21 to both sides
3x+28=5x
subtract 3x from obht sides
28=2x
sivide both sides by 2
14=x

Answer 2

3x + 7=5x-21
28=2x. Add 21 to both sides and subtract 3x on both sides
X=14 divide both sides by 2