f(ax + b)= ax² + 2abx + b² + 4ax + 4b + 3= ax² + (2ab + 4a)x + b² + 4b + 3又f(ax + b) = x² + 10x + 24所以 a = 1 , 2ab + 4a = 10 , b² + 4b + 3 = 24解得: a = 1 , b = 3所以 5a - b = 5 - 3 = 2