7.5+13.6=12.4+[]
work out the missing number

Lets take, Missing number as = x

7.5+13.6 = 12.4+x

21.1 = 12.4+x

21.1 - 12.4 = x

x = 8.7

Re-check:-

=> 7.5+13.6 = 12.4+8.7

=> 21.1 = 21.1

8.7

Step-by-step explanation:

I'm gonna use algebra for this if you don't mind.

7.5 + 13.6 = 21.1

21.1 = 12.4 + x

Subtract 12.4 from both sides:

21.1 - 12.4 = 12.4 - 12.4 + x

8.7 = x

Reasoning:

First add 7.5 and 13.6 to get 21.1.

You'll get something that looks like this: 21.1 = 12.4 + _

We want a number that adds to 12.4 to make 21.1, so we subtract 21.1 from 12.4 and we get 8.7

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'.