5,-1,3, write the first term a and the common difference d

The appropriate formula here is l = a + (n-1)d, where l represents the last term.

Here the first term is -5 and the common difference is +4.  Thus,

l = -5 + (n-1)(4)    (answer)

Check:  we are given the arith sequence -5, -1, 3, 7.    Does our formula correctly predict the 1st term?  l = -5 + (1-1)(4) = > -5.  Yes.
Does it correctly predict the 3rd term?  l = -5 +(3-1)(4) => -5+2(4) = 3.  Yes