We know the equation:

gives the value of the investment after x dollars. This equation has the form:

where m is the slope. Comparing the equation for the value and the linear equation above we conclude that m=3.3,
Since the value of the slope is positive this means that the value is increasing; also we know that the slope is 3.3 but that it is express as thousands of dollars, hence it is increasing at a rate the 3300 dollars. Therefore we have:
The value of the investment is increasing at a rate of 3300 dollars per month.