First find the slope of the first line
L1
m = ( y2-y1)/(x2-x1)
m = ( 1-2)/(3-1)
= -1/2
Now find the slope of the second line
L2
m= ( y2-y1)/(x2-x1)
= ( 0 - -1)/(2 -0)
= (0+1)/(2-0)
= 1/2
Parallel lines have the same slope
These lines do not have the same slope, they are different by a negative sign.
These lines are not parallel.