Let \( f(x) \) be a positive function such that the area bounded by \( y = f(x) \), \( y = 0 \) from \( x = 0 \) to \( x = a > 0 \) is \( e^{-a} + 4a^{2} + a - 1 \). Then the differential equation, whose general solution is \( y = c_{1} f(x) + c_{2} \), where \( c_{1} \) and \( c_{2} \) are arbitrary constants, is:
