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5^3x  =  900  take the log of both sides

log 5^3x  =  log 900      and we can write

(3x)  log 5  =  log 900     

x  (3log 5)  = log 900     divide both sides by 3log 5

log (900) /  [ 3log (5) ]  ≈ 1.41

ln 3 +  ln (4x)  =  4    we can write

ln (3*4x)  =  4

ln (12x)  =  4

This says that

e^4  =  12x         divide both sides by 12

e^4 / 12  = x ≈  4.55