7log(3x)+3log(5x) =15
(3x)^7 * (5x)^3 = 10^15
2187 x^7 * 125 x^3 = 10^15
273375 x^(7+3) = 10^15
x^(10) = 10^15 / 273375 Take 10th root of both sides
x = 9.04324
+37093 
