Question 1111885
These problems you've posted are both very basic.  I have to assume you are just learning logarithms for the first time.   You should expend the energy on your own to really learn this important concept (logarithms).  Spend enough time studying and doing problems until you have mastered it.  

{{{ log(10, (10000)) = 4 }}}      because {{{10^4 = 10000 }}}
{{{ log(10, (1000)) }}} = ___________

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In short, a logarithm is an exponent.      

{{{ log(b, ( N)) }}} is the power to which b must be raised (i.e. the exponent applied to b) to give you back N.   Another way to put it:   if {{{ log(b,( N)) = x }}} then  {{{ b^x = N }}}

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One way logarithms are useful: they  "reduce" the level of operations needed to solve an equation:

    {{{ log(b, (a^x)) = x*log(b,a) }}}     <----<< exponentiation becomes multiplication
    {{{ log(b, (x*y)) = log(b, (x)) + log(b,(y)) }}}    <----<< multiplication becomes addition

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See lessons on logarithms by other tutors.  Here is a good one by tutor Ikleyn on solving real-world problems using logarithms, at the bottom of that lesson she has links to other related lessons:  
https://www.algebra.com/algebra/homework/logarithm/Using-logarithms-to-solve-real-world-problems.lesson