Take the coordinates on each line and find their related slopes:
Slope formula m= (y₂-y₁)/(x₂-x₁)
1st . The two given coordinates of the LOWER line are: (-1,-1) and (1,-2)
them m = (-2+1)/1+1) = -2/2 = -1
2nd. The two given coordinates of the HIGHER line are: (-1,2) and (1,1)
them m = (1-2)/(1+1) = -1/2
3rd. The two given coordinates of the SECANTE are: (1,0) and (2,2)
them m = (2-0)/(2-1) = 2/1 = 1
We notice that the 1st line has m = -1 and the 3rd (secant), m = 1, then only the 1st line and the secant are perpendicular because the product of their slope is equal - 1 (this is the condition to have perpendicular lines)
