Which term of the A.P. 53, 48, 43, …. is the first negative term?

Let us first evaluate which term is 0 because the next term will be the term containing first negative number.

a = 53, d = 48 - 53 = -5, a_n = 0.

By formula, a_n = a + (n - 1)d
⇒ a_n = 53 + (n - 1)(-5)
⇒ 0 = 53 - 5n + 5
⇒ 5n = 58
⇒ n = $11\dfrac{3}{5}$

The next term will be containing the first negative number.

Hence, the 12th term will contain the first negative number.