Question 864059
S = ½(2a + (n-1)d)n
48 = ½(6 + (5)d)6,
168 = ½(6 + (n-1)d)n
d = 2,   n = 12

an = a + (n - 1)d
a12 =3 + (11)2
a12=25
check
S = ½(a + an)n
S = ½(3 + 25)12=168
ok