Answer:
y-3 = -2/15 (x-9) 
Explanation:
First, we should find the gradient, m, which is 

This is 

A linear (straight) line has a general equation y=mx+c 
y = 
x+c 
We can rewrite the equation into the general form: 
y-y1 = m(x-x1)
where y1 is the value of a y coordinate and x1 is the x value of the coordinate that the y1 value was taken from. 
This stems from the idea that 0 = 0. 
To make that equation true, we can make both sides equal to 0. 
To do that, y - y1 has to be - and x-x1 has to be zero. m can be ignored as anything multiplied by 0 is 0. 
Up until here, y1 and x1 are arbitrary constants. They are values that we have to find. However, we don't actually. We have coordinates that the x and y will equal to. (9,3)
Because 9 - 9 = 0 and 3 - 3 = 0, we can state that the value of y1 is 3 and x1 is 9. 
Hence, the equation is y-3 = -2/15 (x-9) 
You can rearrange the equation to make it into the form y = mx + c, but this is usually not necessary. 
I hope this helped.