How do you solve two simultaneous equations? (i.e. 5x + y =21 and x - 3y =9)

There are two ways of solving the simultaneous equations. The easiest one is to write two equations beneath each other and then try to get either x or y values the same on both of them by multiplying the value in front of x (or y) on top equation by the whole bottom equation, and the top equation by the coefficient in front of x of bottom equation. In this case we would get 5x + y =21 and 5x - 15y =45. Then at this stage we can subtract the bottom equation from the top one eliminating the x unknown and leaving the equation in terms of y. i.e. y + 15y = 21 - 45 ==> 16 y = -24 ==> y = -3/2. Then we use the calculated value of y to find x by plugging y into one of the two equations. i.e. x -3(-3/2) = 9 ==> x = 9 - 9/2, x = 9/2. Another approach would be to rearrange one equation in terms of x so x = 9 + 3y and then plug it into the other equation and solving that for y. Then follow the same approach.