He borrowed "a" at 7% and "b" at 8.25%
now, whatever "a" and "b" are, we know, his student loan was 9000, thus
a+b = 9000
now, he owes 7% or 7/100 for "a", what's 7% of a? well, 7/100 * a or 0.07a
he owes 8.25% of "b", how much is 8.25% of b? well, 8.25/100 * b or 0.0825b
now, we know, at the end of the year, he owed for both loans, 706.25 in interest
thus we know that 0.07a + 0.0825b = 706.25
thus
solve for "a", to see how much he borrowed at 7%
what about "b"? well, b = 9000-a
now, whatever "a" and "b" are, we know, his student loan was 9000, thus
a+b = 9000
now, he owes 7% or 7/100 for "a", what's 7% of a? well, 7/100 * a or 0.07a
he owes 8.25% of "b", how much is 8.25% of b? well, 8.25/100 * b or 0.0825b
now, we know, at the end of the year, he owed for both loans, 706.25 in interest
thus we know that 0.07a + 0.0825b = 706.25
thus
solve for "a", to see how much he borrowed at 7%
what about "b"? well, b = 9000-a