Question 57473
Well to set y as a function of x means that y equals something of y

{{{-2y+10x=8}}}
{{{-2y=-10x+8}}}
{{{y=5x-4}}}

Now it becomes y is a function of x because it is in the form of y equals something of x, in this case, 4x-4

If you plug in a number for x, you get a unique y-value.  That is the definition of a function.