Answer
$\approx77.82$
Work Step by Step
From the law of cosines
$20^{2}+x^{2}-2\times 20\times x\times \cos 60^0=70^{2}\Rightarrow x^{2}-20x+400=4900\Rightarrow x^{2}-20x-4500=0$
$\Rightarrow x=\dfrac {20\pm \sqrt {20^{2}+4\times 1\times 4500}}{2}=\dfrac {20\pm \sqrt {18400}}{2}\approx 77.82$
