a = amount invested at 8%
how much is 8% of "a"? (8/100) * "a", namely 0.08a.
b = amount invested at 6%
how much is 6% of "b"? (6/100) * "b", namely 0.06b.
we know the total amount invested is 210000, so whatever "a" and "b" might be, we know that a + b = 210000.
we also know that the yielded amount in interest is the same for both, so whatever 0.08a and 0.06b are, we know they're the same.