title: how to do math operations on a pandas columns and save it as a new dataframe
body: <pre><code>                  a          b           c
  6/29/2018   0.744189037 0.251833984 0.632784618
  6/30/2018   0.476601558 0.694181607 0.7951655
  7/1/2018    0.91376258  0.111712256 0.813708374
  7/2/2018    0.849505199 0.037035716 0.973137932
  </code></pre>
  
  <p>So I am trying to use this data frame and create a new one based off of math operations. So far I have <code>df3 = df2[['a','b']] and df4 = pd.DataFrame(df3[['a']]*2 - df3[['b']])</code>. </p>
  
  <p>To summarize, I want to use df3 and multiple column a by 2 and subtract that value by column b and put the result into a brand new data frame. </p>
  
  <p>The final output that I would want to get too would be</p>
  
  <pre><code>               2*a-b
  6/29/2018   1.23654409
  6/30/2018   0.259021508
  7/1/2018    1.715812903
  7/2/2018    1.661974683
  </code></pre>
  
  <p>However this yields all values of <code>NA</code>. </p>