What is to do when you have to use more than one unknown variable?

We can solve the equation of two unknown variables for one variable in terms of second variable.

Step-by-step explanation:  

We know that, an equation with a variable can be solve easily. But in the case of two unknown variables, we can solve the equation for one variable in terms of second variable. For example:

If we have given an equation with two unknown variable x+y=4, we can solve this equation for variable y in terms of variable x or we can solve this equation for x in terms of y. Which could be written as:  

y=4-x

or

x=4-y.

you didn't give me the choices, so i will just give you the graph so you can look at your choices and choose the right one.