We are given with,

tan^-1 3 + tan^-1 x = tan^-1 8

We need to find the value of x.

Using property of inverse trigonometry,

Let us replace A by 3 and B by x.

Since, according to the question

tan^-1 3 + tan^-1 x = tan^-1 8

So,

Taking tangent on both sides,

Using property of inverse trigonometry,

tan(tan^-1 A) = A

Now, in order to find x, we need to solve the linear equation.

By cross-multiplying,

⇒ 3 + x = 8(1 – 3x)

⇒ 3 + x = 8 – 24x

⇒ 24x + x = 8 – 3

⇒ 25x = 5