Use figure to estimate the phase difference between gx t

Question

Use Figure to estimate the phase difference between g(x, t) and h(x, t) at t₁ = 12.24 ms.

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If the waves shown in Figure 2 are to form a standing wave certain conditions must be met. Which of the following must be true if g(x, t) and h(x, t) are to combine to form a standing wave?