how to solve y=mx+b with coordinate (x, 13)

If that is all the information the problem gives you then I believe it means that y=13 no matter what the value of x. Which means you have a straight horizontal(parallel to the x-axis) line at y=13 for all values of x.

As is, there is not enough info. There should be another coordinate point.

Here is what we have so far:



We can not find slope with one given point, nor can we find b (the y intercept).