let 'x' represent her brother's age, then x^2 is the square of her brother's age

if 'x' represents Gali's age, then 'x+3' represents her brother's age and 'x^2 + 6x + 9' represents the square of her brother's age

...most likely there is an error in the problem statement... "Double the square of her brother's." is an incomplete sentence and it is not clear what should be solved for or if additional information is missing.