Question 139510
You have two unknowns, the number of each stock bought, so you need two equations. 

First, let x=number of stock at $3.50 and y=number of stock at $6.00.

We know that {{{x + y = 500}}} 
and we know {{{3.50 * x + 6.00 * y = 2187.50}}}

Solve the first equation for y: {{{y = 500 - x}}} and substitute this value for y into the second equation.

{{{3.5x + 6(500 - x) = 2187.50}}}
{{{3.5x + 3000 - 6x = 2187.5}}}
{{{3000 - 2.5x = 2187.5}}}
{{{812.5 = 2.5x}}}
{{{x = 325}}} the number of $3.50 stocks bought. 
{{{y = 500 - 325 = 175}}} the number of $6.00 stocks bought.