Algebra time. We start with 42, this is our base number, we cannot go below this one. Since we add 3 chairs every row, then we add 3(x) to every row after the first one. The first row has 42 chairs. The second has 42 + 3(1), and the third row has 42 + 3(2). X is going to be one less than the row number you’re in. So for the 25th row we do 42 + 3(24). And for the one with 90 seats we have 42 + 48 = 90. There’s a difference of 48 chairs between 42 and 90. So we divide 48 by 3 to see the value of x. 48/3 is 16, so the equation is 42 + 3(16) = 90. Therefore we’re on the seventeenth row. An easier way is to start at 39 (42-3) and then do 39 + 3(1) as the first row.
Write out what you see. "Three times" is 3 · something; "the absolute value of the sum of a number and 6" is |number + 6|. We'll use x for our number. Put it all together and you get 3 · |x+6|
