Question 107381
We use Pythagoras' theorem. 

{{{a^2+b^2=c^2}}}

c is your hypotenuse

so we have

{{{7^2+b^2=20^2}}}
{{{49+b^2=400}
{{{b^2=400-49}}}
{{{b^2=351}}}

{{{b=sqrt(351)}}}
{{{b=18.73}}}