Question 38586
You have it set up right, but I think you are confusing yourself with the units of m and m^2.  I suggest that you just solve the equation for the unknown which is b, and apply the units at the end of the problem.  It will look like this:

{{{224 = (1/2)*(16)(20+b) }}}


Now, take the one half times the 16, which is 8.  Now it should begin to look a LOT better.
{{{224 = 8(20+b) }}}


In fact, you can divide both sides by 8, since 8 goes into 224 evenly (28 times!)
{{{224/8 = (20+b)}}}
{{{28= 20+b) }}}
{{{b= 8 }}} meters.


R^2 at SCC