If you are trying to find an equation of a line that has that slope (-3) and passes through the point (-1,3) the easiest way to do this is by writing the linear equation in slope-intercept form, y = m(x-a) + b, where (a, b) is the point through which the line passes and m is the slope.
For your example, m = -3, and (a,b) = (-1,3).
y = -3(x+1) + 3 (remember that a = -1, so x-a becomes x+1). This can be rearranged as desired into other forms.
If you're not trying to find the equation of a line that way, then the question is unclear and you should clarify it.
