$$\begin{array}{rl}& Y_{Actual}=-1115.79+2.54\cdot X_1-2.27\cdot X_2\\ & +383.20\cdot X_3-0.63\cdot X_4+0.04\\ & \cdot X_1\cdot X_2+0.10\cdot X_1\cdot X_3-X_1\cdot X_4\cdot {10}^{-3}\\ & +0.55\cdot X_2\cdot X_3-5\cdot {10}^{-4}\cdot X_2\cdot X_4\\ & +0.14\cdot X_3\cdot X_4-0.11\cdot X_1^2-0.04\cdot X_2^2\\ & -32.90\cdot X_3^2-6.9\cdot {10}^{-4}\cdot X_4^2\end{array}$$ (3)