How to find the slope with only the x and y intercepts?

If we have the equation of a line: y=ax+b

How to find a if with only the x - intercepts (called m) and y-intercepts (called n)

+ First we have y=0 and x=m or 0=a*m+b or b = - a*m

+ Then we have x=0, y=n or n=a*0+b or b=n

And if m≠0 we have a = - n/m

and if m=0 this line has no slope