$\bm{f(x)=5,470(0.64)}^{\frac{x}{2}}$

The function $\bm{f}$ gives the value, in dollars, of a certain piece of equipment after $\bm{x}$ months of use. If the value of the equipment decreases each year by $\bm{p}\%$ of its value the preceding year, what is the value of $\bm{p}$?