Final answer:
The expression $2.1\cdot x\cdot z\cdot z\cdot z\cdot z$ can be written using exponents as $2.1\cdot x\cdot z^4$, which shows that z is multiplied by itself four times.
Step-by-step explanation:
The expression $2.1\cdot x\cdot z\cdot z\cdot z\cdot z$ uses multiples of the variable z. Since z is being multiplied by itself, we can represent this repeated multiplication using exponents. The expression simplifies to $2.1\cdot x\cdot z^4$ where z^4 represents z multiplied by itself four times. This is an application of the rules for multiplication of exponentials, where a number multiplied by itself several times is written with an exponent equal to the number of times it is multiplied.