Question

What does it mean for an equation to express y in terms of x

Answer 1

This is also known as constant of variation. 

So, lets give an example

y = 4 and x = 2

If y = 8, what does x equal?

4*2 = 8
y = 8

x = 2
2*2 = 4

x = 4

We solve for the missing value based on the other value.