how do you calculate the correlation coefficient between two variables

Well, there are two equations for it that I'm aware of and there may be more.

This is the one I originally learned:

r_{xy}\ =\ \frac{\Sigma{xy}\ -\ \frac{(\Sigma{x}\Sigma{y})}{n}}{sqrt{\Sigma{x^2}\ -\ \frac{(\Sigma{x})^2}{n}}\ sqrt{\Sigma{y^2}\ -\ \frac{(\Sigma{y})^2}{n}}}

This is another one in my book. The standard deviations in the denominator are hopefully something you've already learned.

r_{xy}\ =\ \frac{\left(\frac{\Sigma{(x\ -\ \overline{x})(y\ -\ \overline{y})}}{n\ -\ 1\ }\right)}{s_xs_y}

Does your equation look like one of these? Or did you need an equation, or are you having trouble figuring out how to work it out? You need to be a bit more specific with the exact help you need.

(And now if you'll excuse me, I'm worn out from figuring out how to write those. :p At least I did finally learn how to do an x-bar!! )