Edward C. answered 09/16/15
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Caltech Grad for math tutoring: Algebra through Calculus
Your work so far is correct. You just need to finish by making the denominator real by multiplying top and bottom by the complex conjugate which is (25-18i)
[(9i-4)/(25+18i)] * [(25-18i)/(25-18i)]
= [225i-100+162+72i] / [625+324]
= [62+297i]/949
= (62/949) + (297/949)i
This is done to get the number in the standard a+bi form for a complex number