Normal to a curve at point P lying on this curve, by definition, is a line perpendicular to a tangent to a curve at this point (presuming the curve is smooth and has a tangent, otherwise a normal is undefined).
Since OP is a tangent to curve y=loge(x) from origin O to point P lying on this curve, it is also a perpendicular to a normal to a curve at point P.
So, our task is to measure the length of OP.
This is done by Pythagorean Theorem as the distance between two points:
origin O with coordinates (0,0) and
point P on a curve with coordinates (XP,YP), where
YP=loge(XP).
OP=√X2P+Y2P=√X2P+log2e(XP)