Answer :
a1 = -12
a2 = a1+d = -12+d
a3 = a2+d = a1+2*d = -12+2*d
.......
a27 = a26+d = a1+26*d = -12+26*d = 66
so: 26*d=66+12
26*d=78
d=78:26
d=3
therefore: a42 = a1+41*d = -12+41*3 = -12+123=111
a2 = a1+d = -12+d
a3 = a2+d = a1+2*d = -12+2*d
.......
a27 = a26+d = a1+26*d = -12+26*d = 66
so: 26*d=66+12
26*d=78
d=78:26
d=3
therefore: a42 = a1+41*d = -12+41*3 = -12+123=111
