simplify the followingL 2 log5 a - 3log b + log ( a+ b)

First of all, I must say that if you had intended for the first
term  "2log5 a" to mean "2 times log to base 5" and the argument being "a", then please go right
to the last section of this answer: Interpretation 2.

Else, my
interpretation of your question is:

Interpretation
1

2.log(5a) - 3.log(b) +
log(a+b)

ie. All log functions in base
10.

2.log(5a) - 3.log(b) + log(a+b)

= 
log(5a)^2 - log(b^3) + log(a+b)

=  log[ (25a^2)(a+b)
/b^3
]

___

Interpretation
2

2.log5 (a) - 3.log(b) +
log(a+b)

= 2.log5 (a) - log(b^3) +
log(a+b)

= log5 (a^2) + log[ (a+b) / b^3
]

= [log(a^2)]/[log 5]  + log[ (a+b) / b^3
]

= log{ a^[2/(log 5)]} + log[ (a+b) / b^3
]

= log { a^[2/(log 5)] . (a+b) / b^3
}

What a complicated expression, isn't
it?  This leaves us to wonder if this is the original intent of the question. 
:)