The third term of an arithmetic profession is 90 and the eleventh is 50.. what is the common difference?
http://www.mathsisfun.com/algebra/sequences-sums-arithmetic.html
a3 = a1 + 2 d = 90
a11 = a1 + 10 d = 50
-----------------------subtract
a3-a11 = 0 -8 d = 40
so
d = -5