If 7^x = 1343 , then the value of x is?

Answer: Option B

$$\begin{array}{rl}& {\text{7}}^x\text{ = }\frac{1}{343}\\ & \Rightarrow {\text{7}}^x\text{ = }\frac{1}{7^3}\\ & \Rightarrow {\text{7}}^x\text{ = }{\text{7}}^{-3}\\ & \Rightarrow x=-3\end{array}$$
(If bases are equal then their power are also equal)