Question

What is the slope and y-intercept of `9x-3y=21`?

Answer 1

I found:
Slope`=m=3`
Intercept`=c=-7`

Explanation:

If you write it in Slope-Intercept form you'll see immediately the slope `m` and intercept `c`:
`y=mx+c`
isolating `y`:
In your case:
`y=9/3x-21/3`
or:
`y=3x-7`
so that:
Slope`=m=3`
Intercept`=c=-7`