I know one is X-Axis
x axsis
x-axis maybe?
X-axis and x-intercept start with x.
Geometry
eicosohedron
right.
Tangent.
Whole Number
vector, vertex, vertical
Since this sentence is lacking a predicate, it is virtually impossible to answer. "Geometry words that start with r" is a legitimate noun phrase, but is so incomplete that it cannot be answered.
The horizontal x-axis on the Cartesian plane
diameter, diallation