Answer
Eagles: 29 million
AC/DC: 22 million
Work Step by Step
Let e = sales for the Eagles’ album in millions
Let a = sales for AC/DC’s album in millions
e = a + 7
a + e = 51
a + (a + 7) = 51
2a + 7 = 51
2a = 44
a = 22
e = a + 7 = (22) + 7 = 29