Answer
$ŷ=3.206~mm$
Work Step by Step
$ŷ=7.790-0.382x_1-0.587x_2-0.925x_3+0.091x_2x_3$
- Age: 12 years old, that is, $x_1=12$
- Female: $x_2=0$
- From Trois Fontaines: $x_3=0$
$ŷ=7.790-0.382\times12-0.587\times0-0.925\times0+0.091\times0\times0=3.206$