We want to find the value of -a, so that the area between the
green line at -a to the big red line in the middle (the y-axis) 
equals to 0.1844.

The area under the whole normal bell-shaped curve is 1 (one).

The big red line in the middle (the y-axis) divide the area
into two symmetrical parts.  The total area to the right of the
y-axis is 0.5. The total area to the left of the y-axis is
also 0.5, and the part of the 0.5 to the left of the green line
is found by subtracting 0.1844 from 0.5:

0.5-0.1844 = 0.3156 and I will indicate this on the drawing:




If you have a TI-83 or TI-84, which calculates from the far left, find
this:

invNorm(0.3156,0,1) and get -0.4800385254

Or if you use the kind of normal table at this site:

https://math.arizona.edu/~rsims/ma464/standardnormaltable.pdf

which reads from the far left, find the nearest entry in the body of 
the table to 0.3156, which is 0.31561 and read -0.48 in the z-column.

Or if you use the kind of normal table at this site:

https://www.mathsisfun.com/data/standard-normal-distribution-table.html

which reads from the middle, find the value 0.1844 in the body of the
table and see that it corresponds to a z-value of 0.48, but reading
from the center to the left is the same as reading from the center to 
right, so interpret that as -0.48.

Edwin