Answer:
a) 21
Step-by-step explanation:
If the columns are labeled a, b, c, d left to right, it appears that we have ...
The value of M is 21.
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Questions like this require that you try different combinations of operations on the numbers shown to see if you can find a relationship. Here, the left column is a perfect square, so that is a clue. The difference of the numbers in the middle columns is a factor of the number in the right column -- another clue. It takes a certain amount of creative thought and familiarity with arithmetic facts. There is no "step-by-step" for a problem like this.