Let's go:
[tex]a_n = a_1 + (n - 1).r \\ a_6 = a_1 + 5r \\ a_11 = a_1 + 10r \\ a_6 = 25; a_11 = 60 \\ a_1 + 5r = 25 \\ a_1 = 25 - 5r \\ a_1 + 10r = 60 \\ a_1 = 60 - 10r \\ a_1 = a_1 \\ 25 - 5r = 60 - 10r \\ 10r - 5r = 60 - 25 \\ 5r = 35 \\ r = 7[/tex]
I hope I helped you.
