For example, your equation for TU and your equation for UV intersect at the point U(a,b).
So x = a, and y = b are solutions to BOTH equations, since that point lies on BOTH lines.
If you think of the equations being y = 6x-11 and y = 3x-1. The point at which these two lines intersect lies on both lines. And hence the y-coordinate of that point is equal for both lines.
Hence y = y
Hence 6x-11 = 3x-1
3x = 10
x = 10/3
This is the x-coordinate of the intersection (and hence the x coordinate of point U!). If you plug that value of x into either of the two equations, you'll get the y coordinated for U.
Now, if you find the three points T U and V using the method above, then you should find which one has the LOWEST x coordinate, and which one has the HIGHEST x coordinate. The range of possible x values for this triangle is the values in between those two coordinates, including the coordinates themselves!
Let me know what answer you get, I'll let you know if you're right or not.