11. Take the logarithm, base X.
a² - ab = 10
Since we are given that b = a - 2, we can substitute for b to get
a² - a(a -2) = 10
a² - a² + 2a = 10 . . . . . eliminate parentheses
a = 5 . . . . . . . . . . . . . . simplify, divide by 2
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Check: a² = 5² = 25
ab = 5·3 = 15
X^25/X^15 = X^10 . . . . . the answer works