Question 281765
Let the slope of first line be m1 and slope of second line be m2 then 
for both lines to be perpendicular to each other m1*m2 = -1

lets find m1 and m2

m1 = (0 - y) / [1 - (-2)] = -(y)/3
m2 = (1 - 3) / (-1 - 3) = 2/4 = 1/2

m1*m2 = -1
-(y)/3 * 1/2 = -1
-(y)/6 = -1

y = 6