Se logx 25 = -2, então log 5 x é igual a:

Your question is on Spanish but I'll try to answer it
log x 25 = -2 => x^-2 = 25 

we know that x^-2 is the reciprocal value that is also written as (1/x)^2, according to this 

=> (1/x)^2 = 5^2 => 1/x = 5 => x= 1/5

Check   log (1/5) 25 = x => (1/5)^x = 5^2 => 5^-x = 5^2 => -x=2 =>
x= -2    Good luck!!!