Seria asi
<meta charset="UTF-8">
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var multiplo = 8;
document.write("<h1>Aprende la tabla del " +multiplo+ " con nosotros</h1>");
document.write("<br>");
document.write(multiplo + " x " + 1 + " es = " + multiplo * 1 + "<br>");
document.write(multiplo + " x " + 2 + " es = " + multiplo * 2 + "<br>");
document.write(multiplo + " x " + 3 + " es = " + multiplo * 3 + "<br>");
document.write(multiplo + " x " + 4 + " es = " + multiplo * 4 + "<br>");
document.write(multiplo + " x " + 5 + " es = " + multiplo * 5 + "<br>");
document.write(multiplo + " x " + 6 + " es = " + multiplo * 6 + "<br>");
document.write(multiplo + " x " + 7 + " es = " + multiplo * 7 + "<br>");
document.write(multiplo + " x " + 8 + " es = " + multiplo * 8 + "<br>");
document.write(multiplo + " x " + 9 + " es = " + multiplo * 9 + "<br>");
document.write(multiplo + " x " + 10 + " es = " + multiplo * 10 + "<br>");
</script>