great, with the graph and the line equation
y=mx+b
m is the slope and b is, in this case, b= 4 (where the line cross the x=0)
so know we have y=mx+4
we know the line cross over the point x=2 y=0
so if we replace it on the equation we will find the slope
0=m*2+4
-4=m*2
-4/2=m
m=-2
the answer is m= -2