the formula is a_{1}+(n-1) d
so a_{1} would be the first number in the sequence, which would be 13 in problem 9.
13+(n-1)d
then you put in n, which is 10 (it represents which number in the sequence you're looking for, for example 16 is the second number in the sequence)
13+(10-1)d
then you find the difference between each number, represented by d which in this case is 3
13+(10-1)3
13+(9)3
13+27=
40
