y is certainly of the form
y '= p (x) y + r (x) y2 + g (x)
In this case p(x) = 0, r(x) = -a, and g(x) = b e-bx if I am reading the image correctly.
Do you mean you don't know the solution for the second order homogeneous ordinary differential equation which is the result of the replacement of y by
y = \(\displaystyle \frac{u'}{a u}\)
I haven't looked in detail at that equation so I'm not sure whether the substitution will help.