To represent the relationship between Kendra's age and Justin's age in an equation, we know that Kendra's age (13 years) is half of Justin's age (a).
This relationship can be expressed mathematically as:
\[ 13 = \frac{1}{2}a \]
To solve for Justin's age (a), we can multiply both sides of the equation by 2:
\[ 2 \times 13 = a \] \[ 26 = a \]
So, Justin's age is 26 years.
Among the options you provided, the correct equation that represents Justin’s age based on the information given would be:
\[ \frac{1}{2} a = 13 \]
This option corresponds to the last option in your list:
Start Fraction 1 over 2 End Fraction a equals 13.