How many strings of six lowercase letters from the english alphabet contain

a) the letter a?

b) the letters a and b?

c) the letters a and b in consecutive positions with a preceding b, with all the letters distinct?

d) the letters a and b, where a is somewhere to the left of b in the string, with all the letters distinct? rosen, kenneth. discrete mathematics and its applications (p. 436). mcgraw-hill higher education. kindle edition.