a2=a1+d
a4=a1+3d
a2^2=a1a4
a2^2=(a1+d)^2=a1^2+2a1d+d^2
a1a4=a1(a1+3d)=a1^2+3a1d
a1^2+2a1d+d^2=a1^2+3a1d
a1=d=2
an=a1+(n-1)d=2+2(n-1)=2n