а(1) = 3
а(2) =7
а(3) = 11
d = a(2)-a(1) = a(3)-a(2) = 4
a(6) = a(1) +5d
a(6) = 3+5*4=23
a(30)=a(1) +29d
a(30) = 3+29*4=3+116=119
