Observe the equations handed out to you that have both the variable "x" and "y" in them. Note how many times the individual symbols appear in the equation. Keep in mind that each might appear more than once in the equation. For example, consider the equations x -- y = 3 and xy + 3y = 4x. In the first one, the two symbols appear just once, but in the latter they appear more than once.
Put all the quantities with the symbol "y" on the left side of the equal sign and those with the "x" symbol to the right of the equal sign. For example, the equation x -- y = 3 will end up being y = x -- 3, and the second equation, xy + 3y = 4x, will remain the same with the quantity with "xy" placed on the left side of the equation so that you may separate the two variables. For the first equation, "y" is now a function of "x" after this step. In the second, you will have to make sure that all the "x's" appear only on the right and "y" only on the left side of the equal sign.
Factor out "y" from the quantities on the left-hand side of the equation to separate any combined variables in a quantity. For example, separate the "xy" in the equation xy + 3y = 4x by factoring the "y" out of the expression on the left-hand side of the equal sign. The equation will appear in the form y(x + 3) = 4x. Make "y" the subject of the formula by dividing both sides of the equation by (x + 3) to leave y only on the left side. The equation will end up as y = 4x/(x + 3). This finally makes "y" a function of "x" in such an equation as well.