To represent the total cost of Joseph's cellphone data plan for \( m \) months, we can use the expression:
\[ \text{Total Cost} = 21 + 55m \]
Here, \( 21 \) represents the one-time startup fee, and \( 55m \) represents the monthly charge of $55 multiplied by the number of months \( m \).
For \( m = 6 \), you can calculate the total cost for 6 months as follows:
\[ \text{Total Cost} = 21 + 55 \times 6 \]
Calculating the multiplication:
\[ 55 \times 6 = 330 \]
Now, adding the startup fee:
\[ \text{Total Cost} = 21 + 330 = 351 \]
So, the total cost for 6 months is $351.