The given differential equation is:
\[
\frac{d^2 y}{dx^2} + \left(\frac{dy}{dx}\right)^3 + x^4 = 0
\]
- The order of the equation is the highest derivative present, which is \( \frac{d^2 y}{dx^2} \), so the order \( a = 2 \).
- The degree of the equation is the power of the highest derivative term, \( \left(\frac{dy}{dx}\right)^3 \), so the degree \( b = 2 \).
Thus, \( a - b = 2 - 2 = 0 \).