There are three arbitrary constants: \( a, b, c \) in the term \( (a + b)\sin(x + c) \), and \( d \) in the exponential term. However, note that \( a + b \) appears as a single term and not individually. So, effectively, we are eliminating 3 constants: \( a + b \), \( c \), and \( d \). Hence, the order of the differential equation obtained is 3.