Answer:
Line 1: 

Line 2: 

Line 3: 

Line 4: 

Line 5: 

Line 6 : 

Line 7: 0
Line 8: 0
Line 9: 

Line 10: 

Line 11: 0
Line 12: 

Explanation:
Slope of line is given by the formula:

Line 1:
The change in 
 coordinate is zero.
 coordinate is zero.
Therefore slope of line 1 is 
 .
.
Line 2:
Change in 
 coordinate = -4
 coordinate = -4
Change in 
 coordinate = 3
 coordinate = 3
Slope = 

Line 3:
Change in 
 coordinate = 4
 coordinate = 4
Change in 
 coordinate = 3
 coordinate = 3
Slope = 

Line 4:
The change in 
 coordinate is zero.
 coordinate is zero.
Therefore slope of line 4 is 
 .
.
Line 5:
Change in 
 coordinate = 7
 coordinate = 7
Change in 
 coordinate = 3
 coordinate = 3
Slope = 

Line 6:
Change in 
 coordinate = -7
 coordinate = -7
Change in 
 coordinate = 3
 coordinate = 3
Slope = 

Line 7:
Change in 
 coordinate = 0
 coordinate = 0
Slope = 0
Line 8:
Change in 
 coordinate = 0
 coordinate = 0
Slope = 0
Line 9:
The change in 
 coordinate is zero.
 coordinate is zero.
Therefore slope of line 9 is 
 .
.
Line 10:
The change in 
 coordinate is zero.
 coordinate is zero.
Therefore slope of line 10 is 
 .
.
Line 11:
The change in 
 coordinate is zero.
 coordinate is zero.
Therefore slope of line 11 is 0.
Line 12:
The change in 
 coordinate is zero.
 coordinate is zero.
Therefore slope of line 12 is 
 .
.