Question

What does it mean reflection across ×=3

Answer 1

You draw a vertical line on the point (3,0) and a vertical like looks like———>>> | , then whatever shape or point it gives you, you reflect off of that line like if it was a mirror

Answer 2

x = 3 is a vertical line on the coordinate grid which passes through 3 on the x-axis. Imagine  this line to be a mirror  so an object to the left of this line will have an image (its reflection) the same distance  to the right of the line.