The horizontal force in A is solved by summing up moment at B.
$$300 (14) = Ax (14)$$
therefore, $$Ax = 300lbs.$$
Note: If your construction is to be bolted in the wall, the support condition shall be assumed to be pinned connection. It is safe to assume that point B will be hinged and point A to be horizontal reaction only.
or the point A to be hinged and B is horizontal reaction only.
Either way, the value of horizontal reaction at A will still be the same. :)
Edit: Other supplemental info based on OP's comment.
A bolted connection is GENERALLY a pinned connection. Examples of this are in BRIDGE TRUSS connections. This is because rigid connections in dynamic systems are catastrophic as it is prone to extreme fatigue due to constant deformation and back to its original shape..
The vertical reaction can be checked (if you are checking/designing for/vs the shearing of bolts) by distributing the vertical force equally into Ay and By. This is not static problem anymore but more on the design method. Therefore, by designing the shearing strength of bolts, you can assume that BOTH point A and B carries the same amount of shear caused by 300lb load, that is equal to 150lb per joint. This is contrary to the assumption in static check that only one of the two points carry the load.
If you want to get the exact force in each point, you might want to use indeterminate methods in solving for the reactions such as force method, virtual work methods, etc. These usually use displacement as 4th parameter as there will be four unknowns in the system. But taking your question as a simple static problem (because this is a shelf), I suggested the simplest approach to your problem.
Still, by summing up moments at B, the value of Ax remains the same.