Help me determine this this if you can!

Which property was used to simplify the expression? 3c+9 +4c=3c+4c+9

In trapezoid abcd, ac is a diagonal and ∠abc≅∠acd. find ac if the lengths of the bases bc and ad are 12m and 27m respectively.

The stem-and-leaf plot shows the heights in centimeters of teddy bear sunflowers grown in two different types of soil. soil a soil b 5 9 5 2 1 1 6 3 9 5 1 0 7 0 2 3 6 7 8 2 1 8 3 0 9 key: 9|6 means 69 key: 5|8 means 58 calculate the mean of each data set. calculate the mean absolute deviation (mad) of each data set. which set is more variable? how do you know?