how to find the first negative term of a given A.P? both the value of the term and its n value.

Any general term of the AP is given by a +(n-1)d 

And so, for the first negative term, you need to find the least value of n such that a +(n-1)d  <0

Now, if you know the value of a and d for the AP, you can substitute them in the above equation and find the value of n which will satisfy the inequality. 

 

Now, that will give you the n value and that can be substituted back into a +(n-1)d  to give the value of the term.