Given that 18, a, b, - 3 are in A.P.

Now a - 18 = b - a  (Common difference of an AP is always same)

=>  a + a - b = 18

=> 2a - b = 18 .............1

Again

      a - 18 = -3 - b            (Common difference of an AP is always same)

=> a + b = 18 - 3

=> a + b = 15 ........2

add eqaution 1 and 2, we get

      2a - b + a + b = 18 + 15

=> 3a = 33

=> a = 33/3

=> a = 11

Put value of a in equation 2, we get

      11 + b = 15

=> b = 15 - 11

=> b = 4

So value of and b are 11 and 4 respectivily.