[tex]l = 7.4 \: \: dm = 74 \: \: cm \\ a = 56 \: \: cm \\ b = ?\\ l = \frac{a + b}{2} \\ a + b = 2l \\ b = 2l - a = 2 \times 74 - 56 = \\ 148 - 56 = 92 \: \: cm = 9.2 \: \: dm[/tex]