is this statement true: a is positive and b is positive so b-a is negative?

If  a = 6  and  b = 9, then  b - a  =  9 - 6  =  3  (positive)

If  a = 6  and  b = 3, then  b - a  =  3 - 6  =  -3  (negative).

Thus, the statement is not true for all possibilities.