Question

How to find the slope of a line with one number given `y=1.5` or `x=-7`?

Answer 1

There are an infinite number of (straight) lines through a single point and therefore there is no single value that can be established as a slope.

Perhaps the intent was that `1.5` was the y-intercept and `(-7)` was the x-intercept. This would give us two points, `(0,1.5)` and `(-7,0)` and the possibility of calculating a slope. Please re-post with correction if this is the case.