Question 34317
The Pythagorean theorem.  I am assuming that you want to know how to use it.  If you want to know how to derive the Pythagorean theorem please see this website: http://www.mathguide.com/lessons/Pythagoras.html

{{{a^2+b^2=c^2}}} is the Pythagorean theorem.
Using the values you provided:
{{{20^2+b^2=29^2}}}  Now you want to solve for b, you can solve this in the same way you solve other equations with variables (eg. {{{x^2+15^2-78^2}}}).  We can take the square root of the whole equation to make it simplerand so we solve:
{{{sqrt(20^2+b^2)=sqrt(29^2)}}} 
{{{20+b-29=29-29}}}
{{{20+b-29=0}}}
{{{20+b-b-29=0-b}}}
{{{20-29=-b}}}
{{{b=-20+29}}}
{{{b=21}}}

Hope this helps,
lyra