\left\{
\begin{array}{l l}
28=sqrt{a b}\\
40=sqrt{(20 a)^2 b^2}\\
\end{array} \right.

The second equation can be written as:


40 = \sqrt {(20a)^2b^2} = 20ab


From the first equation you have:


28 = sqrt{ab},\\
28^2 = ab


So combining:


40 = 20ab = 28^2\\
40 = 784


It appears that you have two equations that are inconistent. Please check the first two eqautions as written to verify.

Oh, sorry, there should have been "+" between all the terms, but it seems there is a problem with the code.

So, the first equation should have been 28 = sqrt (a + b) and the second one 40 = sqrt ( (20 + a)^ + b^2)