a, 5^x . (5^3)^2=625
5^x . 5^6=5^4
5^x=5^4:5^6
5^x=5^-2
=> x=-2
b)(5.x+1)2=\(\frac{36}{49}\)
\(\Rightarrow\)5.x+1=\(\sqrt{\frac{36}{49}}\)=\(\frac{6}{7}\)
\(\Rightarrow\)5.x=\(\frac{6}{7}\)-1=\(\frac{-1}{7}\)
\(\Rightarrow\)x=\(\frac{-1}{7}\):5=\(\frac{-1}{35}\)
Vậy x=\(\frac{-1}{35}\)
Chúc bạn học tốt!