y = Y(x) and z = Z(x) simultaneously satisfy the system of equation:

y' = z, z' = x^3 (y = z)

The problem gives you initial conditions and initial guesses (for successive approximation). But my question is how can I transform this into y wrt x and z wrt x without going to second order. Because the formula that I know for doing approximation involves only two variable and only to the first order.