Finding roots of an exponential equation with multiple base numbers

Hey everyone, first post.

Just wondering if anyone can help me out with finding 'a' when the equation below is equal to 0

[IMG]file:///C:/Users/Jaiba/AppData/Local/Temp/moz-screenshot-2.jpg[/IMG][IMG]file:///C:/Users/Jaiba/AppData/Local/Temp/moz-screenshot-3.jpg[/IMG](94-10^a)*10^a*ln10+(118-16^a)*16^a*ln16+(147-25^a)*25^a*ln25+(180-40^a)*40^a*ln40+(230-60^a)*60^a*ln60

Cheers

[IMG]file:///C:/Users/Jaiba/AppData/Local/Temp/moz-screenshot.jpg[/IMG][IMG]file:///C:/Users/Jaiba/AppData/Local/Temp/moz-screenshot-1.jpg[/IMG]