1)
y1 = 2x + b
y2 = -2x - 1
You do not have b so there is an infinity of solutions (y1 could be all the parallel lines with slope 2).

2)
y1 = 2x
y2 = mx - 1
This time you have y1 but you do not know the slope of y2, you do not know the direction of y2 so that is the same you have an infinity of solutions.

1) and 2)
y1 = 2x
y2 = -2x - 1
You have the equations of the two lines y1 and y2 so it is ok you know where they intersect.

That is C, you need both.

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If you want to find it
2x=-2x-1
4x=-1
x=(-1/4)

And for the y you put x=(-1/4) in the equation of y1 or y2 that is the same result given they intersect at this point:
2*(-1/4)=-1/2
-2*(-1/4)-1=-1/2

They intersect at (-1/4;-1/2)