Just use the point slope formula:

y - y1 = m(x - x1)
where (x1,y1) = (1,2)

(a) using m = 3:





That's this red line:



(a) using m = 
Multiply both sides by 3




That's this green line:



They're perpendicular because their slopes are negative reciprocals,
which is the same as saying that when you multiply their slopes
together, you get -1.  
 
Edwin