Evaluate the vector-valued function at each given value of t. (If you need to use At, enter Deltat.) r(t) = _thi - (t - 4)j 2 (a) r(4) = 8i (b) r(0) = 4j ( c) r(s + 4) = 7 ( s + 4 ) 2 - sj (d) r(2 + At) - r(2) = At ( 3 + At)i - Atj X