Question 230071
First you need to find the slope using the formula m=(y2-y1)/(x2-x1).  I will leave you to do the arithmetic but you should find that the slope is 1.  
So now we plug into the equation y=mx+b and we solve for b.  Again I will leave you to plug in the points and solve but weither you plug in (9,9) or (-6,-6) you will find that b = 0.  

So now we know that the equation of the line is y = mx+b and we know m and b so plugging in we get y = 1(x)+0 or y = x.