$$ \begin{cases} F&=750\ut{N}\\ k&=2.5\times10^5\ut{N/m}\\ \end{cases} $$ $$\ab{a}$$ $$F=kx,$$ $$ \begin{aligned} x&=\frac{F}{k}\\ &=3.0\times10^{-3}\ut{m}\\ &=3.0\ut{mm}\\ \end{aligned} $$ $$\ab{b}$$ $$ \begin{aligned} W&=\frac{1}{2}kx^2\\ &=\frac{1}{2}k\(\frac{F}{k}\)^2\\ &=\frac{F^2}{2k}\\ &=\frac{9}{8}\ut{J}\\ &=1.125\ut{J}\\ &\approx 1.1\ut{J} \end{aligned} $$ $$\ab{c}$$ $$F=kx,$$ $$\ab{d}$..