Formulas used :
[tex]loga^ x = x \ log a[/tex]
[tex]logab = log a + logb[/tex]
Steps :
[tex](x+y)^2 = 125xy\\\\log(x+y)^2 = log(125xy)\\\\2log(x+y) = log 125 + log x + log y\\\\2log(x+y) = log 5^3+ log x + log y\\\\2log(x+y) = 3 log 5 + log x + log y\\\\[/tex]
