(a_5=21), (S_6=90)

[ a+4d=21\tag{1} ] [ \dfrac{6}{2}[2a+5d]=90\implies 3(2a+5d)=90\implies 2a+5d=30\tag{2} ]

From (1): (a=21-4d). Into (2): [ 2(21-4d)+5d=30\implies42-8d+5d=30\implies42-3d=30\implies3d=12\implies d=4 ] [ a=21-16=5 ]

[ a_{18}=5+17\cdot4=5+68=73 ]

Answer: (73)