The solution can be determined if we can specify the value of the arbitrary constant C₁. We do this by using the initial condition. When x=0, y is equal to -18. Substitute this to the general solution.
y = 1/(1 + C₁e⁻ˣ)
-18 = 1/(1 + C₁e⁰)
-18 = 1/(1+C₁)
C₁ = -1- 1/18 = -19/18
Therefore, the specific solution is:
y = 1/(1 - 19e⁻ˣ/18)