Let the number of rows be x
Then the number of columns will be 2x
Then the number of coins will be (x)(2x)

Then (x)(2x) ≧ 175
         2x² ≧ 175
Divide both sides by 2:
          x² ≧ 87.5
Take positive square roots of both sides:
           x ≧ √87.5
           x ≧ 9.354143467S

So the smallest whole number x can be is 10

So there would be 10 rows and 20 columns.
That would be 10×20=200 coins.

So we'd need 200-175 or 25 more coins.  

Edwin