Penrose gives this exercise his "very straightforward" rating and in my solution I just use the expression which is the subject of exercise 4.4 and simple algebra to show that the expression on page 79 follows from this.

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I just noticed that my solution is very similar to AlvaroJ's solution in the exercise discussion forum. The trouble with the solution obtained by substituting ix for x is that it gives no insight into what is going on, which AlvaroJ's solution and my solution do.