A certain spring has a spring constant k1 = 660 N/m as the spring is stretched from x = 0 to x1 = 35 cm. The spring constant then changes to k₂ = 250 N/m as the spring is stretched to x₂ = 65 cm. From x₂ = 65 cm to x₃ = 89 cm the spring force is constant at F₃ = 105 N.

Write an equation for the work done in stretching the spring from x₁ to x₂.