I'll do a different problem that's worked exactly the same way, so
you won't be getting credit for what somebody else did. I'll do this
one instead:

y - y1 = m(x - x1)

y - 9 = -3(x - 5)

y - 9 = -3x + 15
    
    y = -3x + 24

To use function notation, substitute f(x) for y

 f(x) = -3x + 24

Now do yours the exact same way.

Edwin