In an arithmetic progression, the sum of the first n even number terms exceeds the sum of the first n odd number terms by 3n. Find the common difference of this series.

my working:

Sigma a(2r) - Sigma a(2r-1) = 2 . upper limit of sigma - n , lower limit - 1

Note that 2r and 2r-1 are both subscripts.

I have no idea what to do next, help me please, thank you!