What property allows you to conclude that 5x+7x=12x

Since both of the numbers have a variable which is x ready to be multiplied, there is no expression leading to the variable therefore you will add normally but you will need to put the missing variable which needs to be multiplied. So overall you can just add and once you get your variable you can be abe to multiply and get your final answer.

Hope this helps

The circle is centered at (-8,3) and has a diameter of 20.

Step-by-step explanation:

The radius is 10 because you would take the square root of 100. To find the diameter you would multiply the radius by 2.