i) 2^x*3^(x+1)=108 <=> 3*2^x*3^x=108 <=> 2^x*3^x=36 <=> 6^x=6^2 <=> x=2.
j) 3^(x+1)*5^x=675 <=> 3*3^x*5^x=675 <=> 3^x*5^x=225 <=> 15^x=15^2 <=> x=2.
k) 3^(x+2)*7^x-3^x*7^(x+1)=2940 <=> 9*3^x*7^x-7*3^x*7^x=2940 <=> 9*21^x-7*21^x=2940 <=> 2*21^x=2940 <=> 21^x=1470.
Cum 21^x este impar, oricare ar fi x natural, iar 1470 este par => ecuatia 21^x=1470 NU are solutii in N.
