Valeria is playing with her accordion. The length of the accordion $A(t)$ (in $\text{cm}$) after she starts playing as a function of time $t$ (in seconds) can be modeled by a sinusoidal expression of the form $a\cdot\cos(b\cdot t)+d$. At $t=0$, when she starts playing, the accordion is $15\text{ cm}$ long, which is the shortest it gets. $1.5$ seconds later the accordion is at its average length of $21\text{ cm}$. **Find $A(t)$.** *$\textit{t}$ should be in radians.* $A(t) = $ [[☃ expression 1]]