For that question, you can use the cosine rule, where; [tex]{a}^{2} = {b}^{2} + {c}^{2} - 2(b)(c) \cos( \alpha ) [/tex] In this case, we have.. [tex] {bc}^{2} = {1.8}^{2} + {2.1}^{2} - 2(1.8)(2.1) \cos(70) [/tex] Solving for BC with this will get you 2.250406, rounded off to the tenth is 2.3.