Ineed , arithmetic sequences always mess me up

Missing term of the sequence is 399.

Step-by-step explanation:

In the given table we will try to find the sequence in terms of y and x parameters.

Since y = 3, 12, 21, 30, it's an arithmetic progression then we can represent nth term of this sequence as y = a + (x-1)d where a = first term, x = number of terms and d = common difference.

y = 3 + (x-1)×d = 3 + (x-1)×9 = 3 + 9x -9 = (9x - 6)

Now we have to find the term x = 45 ( 45th term)

By putting the value of x = 45 in the equation we will find the value of y.

y = 9×45 - 6 = 405 -6 = 399