The idea is covering half of the text with a \colorbox{} that's filled entirely by white space. More about colorboxes at

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text boxes with color and . The whitespace is achieved by using the horizontal space command \hspace{}, and the 18em inside the bracket says how many em nits of horizontal space there should be (increasing the number will make the box wider, decreasing it will make it not-wide-enogh to cover the text at this fontsize).