Holding the spring constant fixed makes the total energy scale with amplitude squared. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Two-metre amplitude stores four joules
With spring constant 2 newtons per metre and center position, amplitude 2 m gives total energy 4 J.
E=21kA2=21×2×22=4
Doubling amplitude makes four times the energy
With spring constant 2 newtons per metre and center position, amplitude 4 m gives total energy 16 J.
E=21kA2=21×2×42=16
Three times the amplitude makes nine times the energy
With spring constant 2 newtons per metre and center position, amplitude 6 m gives total energy 36 J.