What is the equation of the line that passes through the point (6 3) and has a slope of -1 over 3?

A line through point (X, Y) with slope m has equation:

y - Y = m(x - X)

The line through (6, 3) with slope -1 has equation:

y - 3 = -1(x - 6)

→ y -3 = -x + 6

→ y + x = 9