Equation of a line problem

[ruby_duby]

equation of a line problem!

The points A and B have co-ordinates (3,4) and (7,-6) respectively. the straight line S passes through A and is perpendicular to AB.
FInd an equation for S, giving your answer in the form ax+by+c=0

Ok so I am really stuck on this question. So far i have calculated the gradient of the line AB by doing (-6-4)/(7-3) and i got -2.5

But now i don't know what to do please help

 

[neutrino]

How are the slopes of perpendicular lines related?

 

[ruby_duby]

i have no idea how the slopes are related

 

[cristo]

Two lines with gradients m and m' are perpendicular if (m)(m')=-1

 

[ruby_duby]

so would that mean that my gradient is +2.5

 

[neutrino]

Would it?

If m' x (-2.5) = -1, what does m' = ?