Explanation:
what is B in your question ? you did not tell us.
since you mention "fraction", I assume B is the slope of a line ?
if that is the case, please remember, the slope is (y coordinate change / x coordinate change) when going from one point of the line to another.
the way you phrased it I assume the slope was originally negative.
that means either the y difference or the x difference was negative (but not both, otherwise it would have been a positive fraction again).
let's look at what happens, when things get reflected across the y axis :
things go from the left side (negative x values) to the right side (positive x values) of the y axis or vice versa.
and things stay on the same side of the x axis (either above the x-axis with positive y values, or below the x-axis with negative y values).
so, that reflection causes the x values to flip their sign, but the y values stay the same.
so, if y was originally negative, then x must have been positive. after the reflection x is now negative too, that makes the y/x fraction -/- = +/+ positive.
if y was originally positive, then x must have been negative. after the reflection x is now positive too, that makes the y/x fraction +/+ positive.
therefore, in every case, the reflected slope fraction must be positive.