Explanation:To find the x substitute 0 in for y,7x+3*0=21.Next,multiply 3 and 0,and that gives you 0,7x+0=21 .After you do that you need remove 0 because when you add and subtract zero the quantity doesn’t change,7x=21.Lastly,divid both sides of the equation by 7,and that gives you x=3.
