Question 417575
3x+7=21

Isolate variable by subtracting 7 from both sides

3x+7=21
  -7=-7

3x=14

divide both sides by 3

3x/3=14/3

x=14/3  therefore 4 is not a solution

check:

3(4)+7=21
12+7=21
19 does not equal 21 so therefore 4 is not a solution