Because the arithmetic square root of 3a+b- 1 is 4, 3a+b- 1= 16 b=2.

You seem to have missed a C. I thought the same thing.

Because c is the integer part of the root number 13, and the root number 13 lies between the root number 9 (that is, 3) and the root number 16 (that is, 4), the integer part of the root number 13 is 3.

So the arithmetic square root of a+2b-c=6 a+2b-c is the square root of 6.