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

Rearrange x - 3y = 9 to make x the subject by adding 3y to both sides of the equation, giving x = 9 + 3y. Substitute the value of "x" in the second equation for "9 + 3y", giving: 5(9 + 3y) + y = 21. Multiply out the brackets to give 45 + 15y + y = 21. Add together like terms: 45 + 16y = 21, then subtract 45 from both sides: 16y = -24, and then divide both sides by 16 to give the value of y = -1.5.
Then substitute the value of y into equation "5x + y = 21" which gives 5x - 1.5 = 21. Add 1.5 to both sides to give 5x = 22.5, and then divide both sides by 5 to give the value of x: x = 4.5.