12: This one if fairy simple: all you need is a line that is not parallel to the line provided, and combined with it, it will provide a solution that's on that line. For example:
y = 2x
x - y = 10
13: Just find two lines that cross (6, -4).
You can use point-slope form:
y - y1 = m(x-x1)
y + 4 = m(x - 6)
then choose two equations with different m's
y = 2(x-6) - 4
y = 3(x-6) - 4
14:
Line one has a slope of two and an intercept at y=0 so it is y = 2x
Line two has a slope of one and an intercept at y=-1 so it is y = x - 1
To find the solution of the system, substitute y:
2x = x - 1
x = -1
Then plug back in:
y = 2 * -1 = -2
So the solution is: (-1, -2)