Question 50481
The gradient of the line between the two points given is found using the formula:
{{{m=(y[2]-y[1])/(x[2]-x[1])}}}
where 'm' is the gradient of the line, and:
(x1,y1)=(4,7)
(x2,y2)=(9,3).
now plug those values into the formula I just gave you:
{{{m=(3-7)/(9-4)}}}
{{{m=-2/5}}}
So the gradient of that line is -2/5. 
All you need to do to find the gradient of a line perpendicular to another is divide minus one by the known gradient. Tis is easier to explain with another formula. Let 'p' be the gradient of the line perpendicular to the line with gradient 'm':
{{{p=-1/m}}}
Its that simple!
Lets put in the value of 'm' that we found earlier:
{{{p=-1/(-2/5)}}}
{{{p=5/2}}}
You might notice that if 'm' is a fraction, all you need to do to find 'p' is turn the fraction upside-down and  change its sign.

I hope this helps. I also run a tuition website of my own. Check it out at www.geocities.com/quibowibbler

Adam