Question

The vertices of a triangle are P(−3, −4), Q(3, 4), and R(−6, −3). Name the vertices of Rx = 0 (PQR).

Answer 1

I want to believe you mean we find the vertices of the vertices P(−3, −4), Q(3, 4), and R(−6, −3) under a reflection on X=0.

Line X=0 is the y-axis. So for reflection under this axis the y coordinates does not change but the x coordinates only change the sign. 
For example, for a point A(x, y) its image under RX=0, A'(-x,y)

A(x, y)  ⇒ A'(-x, y)
P(−3, −4)    ⇒ P'(3, -4)
Q(3, 4)        ⇒ Q'(-3, 4)  and
R(−6, −3)   ⇒ R'(-6, -3)