Tierra is making steps for a doll house. She has cut a piece of wood that measures 1.5 centimeters wide and 30 centimeters long. Each step will measure 15 millimeters by 4
millimeters. She will make 21 steps for her doll house. Which are the dimensions of
the wood that remains after Tierra finishes making the 21 steps?

207/8 is the right

for the friend request.

when a figure, such as triangle (xyz) is reflected over the line y = x to create x'y'z' the x-coordinates and y-coordinates of the original image (pre-image) will be reversed for the image. (e.g., (x, y) > (y, x)) this means that the pre-image position of the x- and y-coordinates switch places which in turn provides a mirror like image of the original xyz.  

i notice that when i draw a line segment from x to x', that the points are parallel from each other -- they are exactly across from each other and do not intersect with any other points.  

yes, think i would see the same characteristic if you drew the line segment connecting y with the reflecting line and then y' with the reflecting line because of the essence of reflections. because it is a mirror image, each point was effected the same way which leads me to conclude that they same would apply to y and y'.