Explanation:
the slope intercept form is
y = ax + b
the slope (a) is always the factor of x in such a normalized form. and we know a = 1.
since we have the slope and a point we can start with the point-slope form
y - y1 = a(x - x1)
where (x1, y1) is a point on the line
and then transform :
y - -6 = 1×(x - -9)
y + 6 = x + 9
y = x + 3