[tex]\displaystyle\\
4)\\
(3^{21}+3^{20}+3^{19}):39=\\\\
=(3^{18+3}+3^{18+2}+3^{18+1}):39=\\\\
=(3^{18}\times3^3+3^{18}\times3^2+3^{18}\times3^1):39=\\\\
=3^{18}(3^3+3^2+3^1):39 =\\\\
=3^{18}(27+9+3):39 =\\\\
=3^{18} \times 39:39 = 3^{18}=3^{9\times2}=\boxed{\bf \Big(3^9\Big)^2 = pp}\\\\\\
6)\\
7^x + 7^{x+2}=2450\\\\
7^x + 7^x\cdot 7^2=2450\\\\
7^x(1+ 7^2)=2450\\\\
7^x(1+ 49)=2450\\\\
7^x \cdot 50 = 2450\\\\
7^x = \frac{2450}{50}\\\\
7^x = 49\\\\
7^x = 7^2\\\\
\boxed{\bf x = 2}
[/tex]
