-10 is equal to 5 subtracted from a number
Assume that the number is x
So the equation will be

we put x at first because in subtraction the number after the word "from" comes first
Now let us find x
We need to move 5 to the left side to find x, so add 5 to both sides

The number is
I am pretty good at box and whisker plot I can help you.
Since the wires are going to make an X, if one angle is 72 deg, then you know the angle directly opposite to it is also 72 deg. Looking at a single side of the x, you see there is a total 180 deg angle (72 + unknown angle), so the unknown angle is 180 - 72, and the angle directly opposite to it will be the same.