Answer
$28 = 2\cdot 2\cdot 7$
Work Step by Step
The $\textit{prime factorization}$ of a number is obtained by writing the number as a product of primes.
To determine the prime factorization of $18$ we write the number as a product of factors and continue the process until all factors are prime numbers.
We start by writing $28$ as a product of two numbers:
$28=2\cdot 14$.
The number $2$ is prime, but $14$ is not. So we write $14=2\cdot 7$:
$28=2\cdot 2\cdot 7$.
Now each factor is a prime number, therefore the prime factorization of $28$ is $2\cdot 2\cdot 7$.
