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