Let me know if you know how to do this

The median of a data set is the measure of center that is found by adding the data values and dividing the total by the number of data values that is the value that occurs with the greatest frequency that is the value midway between the maximum and minimum values in the original data set that is the middle value when the original data values are arranged in order of increasing (or decreasing) magnitude

Me complete this proof! prove that a quadrilateral is a square. me with the steps for this proof.

What is the value of x when x + 7 = 21?