3y + 4x = 8 represents a line on the x-y plane, looking like the image I attached.
To find the x-intercept means to find the place where the line intersects the x-axis. If the line is intersecting the x-axis, then the point where it intersects the x-axis must be a place where y = 0. If y didn't equal 0, then that point wouldn't be on the x-axis.
So, to find the x-intercept, we set y in the given equation to 0, like this:





So,

. We assumed that

to find the x-intercept at the beginning. Then, our ordered pair in the usual order

is

. You can see this is correct by looking at the attached graph.