Question

Is 3x+8y=5 a function? I am thinking yes

Answer 1

the equation describes a function, yes.

if you want to write y as a function of x, then you have to rearrange things a bit:

y = (5-3x)/8

Answer 2

Well, do you know what a function is?

A function is an equation where every input has only one output.

In notation, x is the input. So the question is really asking: "Can x equal anything in following equation which will result in multiple y-values?"

The best way to figure this out is to isolate y:

3x + 8y = 5
3x - 3x + 8y = 5 - 3x
8y = 5 - 3x
y = 5/8 - 3/8x

So, can we get multiple y-values given any x? In this case, the answer is no, so
3x + 8y = 5 is a function.