Mark M. answered 08/26/21
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Retired math prof. Calc 1, 2 and AP Calculus tutoring experience.
9i + 3j = a(i + j) + b(i - j)
9i + 3j = (a + b)i + (a - b)j
a + b = 9
a - b = 3
2a = 12
So, a = 6 and b = 3