To calculate the cost after the discount for Brianna's purchase, we start with the total cost before the discount, which is denoted as \( cc \).
Since Brianna has a 10% discount, we first need to determine the amount of the discount. The discount amount can be calculated as:
\[ \text{Discount} = 0.10 \times cc \]
To find the total cost after applying the discount, we subtract the discount amount from the original total cost:
\[ \text{Cost after discount} = cc - \text{Discount} \]
Substituting the discount into the equation gives:
\[ \text{Cost after discount} = cc - (0.10 \times cc) \]
This simplifies to:
\[ \text{Cost after discount} = cc(1 - 0.10) \]
Calculating \(1 - 0.10\) we get \(0.90\), thus:
\[ \text{Cost after discount} = 0.90 \times cc \]
Therefore, the expression that represents the cost after the discount is:
\[ 0.90cc \]