How do I solve simultaneous equations? eg 1) 4x = 16 - 2y and 2) 3x + y = 9

With simultaneous equations it's important to write down all your working so you keep everything clear in your mind and don't get confused. Start by putting all the x and y of both equations on one side. In this case 4x = 16 - 2y -> 4x + 2y = 16, 3x + y = 9. Now look for terms that have the same coefficient (+ or - doesn't matter). In this case, none do. We could multiply equation 2) by 2 so both have 2y, but equation 1) nicely divides by 2 to give: 4x + 2y = 16 -> 2x + y = 8.
Now we have two equations with +y, so we subtract one equation from the other to get rid of the y terms. It's easiest to work this out if you write the equations one above the other, but we have 3x + y - (2x + y) = 9 - 8. This gives us x = 1, which is half of our solution. Next we substitute this answer into one of the equations, eg 3x + y = 9. If x = 1 this is 3 + y = 9, so y = 6.