The index form of 343 is written as 73.
To explain this, we can look at how we derive the index form. The number 343 can be expressed as a power of a prime number. When you break down 343, you find that it is equal to 7 multiplied by itself three times: 7 × 7 × 7. In mathematical terms, this can be written using exponents, where 7 is the base and 3 is the exponent. Thus, the index form of 343 simplifies to 73.