Solve the simultaneous equations 5x + y = 21 and x- 3y = 9

There are different ways to approach solving a simultaneous equation question. For this one I recommend using substitution for this one where you insert one equation into the other.
First take the equation x - 3y = 9 . Rearrange that so you get x equal to something, in this case it would be x = 9 + 3y by adding the 3y to both sides. Now that we have a value for x we can put it into our other equation, 5x + y = 21, which becomes 5(9+3y) + y = 21. Now expand the bracket to get 45 +15y +y = 21 which is now an equation we can solve. Rearrange so we get all y values on one side of the equals and everything else on the other side: 16y (adding the 15y and y) = -24 (21 -45). We then divide by 16 to get a value for y which will be -1.5. Getting a value for x is a lot easier as we just substitute our y value into the first equation, x = 9+3y , becoming x = 9 + 3(-1.5). Therefore our x value will be 4.5.