These are linear systems. You can solve them with two methods : substitution and elimination. I hate the last one so I am going to explain substitution. It always works for linear systems and is quite easy and nice.
Take your first system :
In the second equation, you know that . Therefore, in the first equation, you can change to . Which gives :
You now know x, substitute it back into the second equation to find y :
Therefore the solutions are Nice, eh ? (the brackets should be braces but I can't get them on the LaTex editor).
Sometimes you will have to work a bit an equation so that one unknown is alone on a side of the equation (y = ...x... or x = ...y...) so that you can substitute easily.
The three last systems, read your lessons on line equations and it should be easy to write a system of two equations and then solve it by substitution/elimination (the last one consists of adding or substracting the two equations to get rid of one unknown, but it doesn't always work).