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

To solve, begin by multiplying both sides of the first equation by 3. This will make the coefficients of y in each equation an equal value of 3. With 15x + 3y = 63 and x -3y = 9, we can now simply add the equations together to remove the unknown y.This gives us 16x = 72.This makes x equal to 72/16. We can simplify this fraction by dividing both the numerator and denominator by 8, giving us 9/2 or 4.5.To solve for y, we just substitute this value into either of the two initial equations, for example the second one. This gives 4.5 - 3y = 9We can subtract the 4.5 from 9 on the right hand side, to get -3y = 4.5Then divide through by -3, 9/2 divided by -3 = -3/2 or -1.5.Now we have both answers, x = 4.5, y = -1.5