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

2 unknowns? Let's make it into a simpler equation you know how to solve with only one unknown! How? Key is to note that you can add and subtract equations. You can also multiply the equations by some constant.Let's start by trying to find y. Notice that we can multiply the second equation by 5, so that we have the same amount of x's in each equation. Then we can subtract the two equations to get rid of the x's.So the initial equations are 5x + y = 21, and x - 3y = 9. We multiple the second equation by 5 so that we have the same amount of x's. This gives us: 5x + y = 21, and 5x - 15y = 45. Now we can subtract the equations to get rid of the x's. So we get 5x + y - 5x + 15y = -24. Simplify this to get 16y = -24. Isolate the y to get y = -24/16. We can then simplify the fraction by dividing the nominator and denominator by 8 to get y = -3/2 or -1.5. Ok great, now we have the value of y!How do we get the value of x? Now that we have the value of y, we can substitute this value into one of the original equations. This gives us an equation with only one unknown (the x), and we know how to solve that to find x.Let's substitute the y value into the first equation. This gives us 5x + (- 1.5) = 21, so 5x - 1.5 = 21. Isolate the x to get x = 22.5/5. Simplify the fraction: x = 45/10 = 4.5.So the answer is y = -1.5 and x = 4.5.