The sum of 11 of an a.p is 891 find the 28 and 45 if the common difference is 15

Answer 1

Poor wording.
The sum of 11 what? 11 terms? I will assume that

You must know your 2 main formulas for an AP
term(n terms) and sum(n terms)

sum(11) = 11/2(2a + 10d), but you told me that sum(11) = 891 and d = 15
so let's sub that into the above
891 = (11/2)(2a + 150)
891 = 11(a + 75)
891 = 11a + 825
take over to find a, then use your definition of term(28) and term(45) to finish up