How do you isolate variable "a" in this equation in PHP : log(20) = log(a) + 3b?

My highschool years are far behind开发者_如何学Python and I'm having trouble remembering how to isolate the "a" variable in this equation :

ln(20) = ln(a) + 3 * b

b = 0.4605

From this website (last section, at the very bottom http://mathonweb.com/help_ebook/html/expoapps_3.htm) : "Back-substituting b into either of the previous equations gives ln (a) = 4.377, and anti-logging gives a = 79.6"

I know this is fairly simple and I just need a quick help!

Thanks a lot for your time

Joel

Not a programming question, but

log(20) = log(a) + 3
log(20) - log(a) = 3
log(20/a) = 3
20/a = 10^3
20/(10^3) = a

// EDIT

You're right, there was a mistake, it's fixed now. Also, b = - 0.4605

Here's the original method, using e instead of log:

20/(e^(3*(-0.4605))) = 79.6173691