Valeria is playing with her accordion. The length of the accordion A(t)A(t)A, left parenthesis, t, right parenthesis (in \text{cm}cmstart text, c, m, end text) after she starts playing as a function of time ttt (in seconds) can be modeled by a sinusoidal expression of the form a\cdot\cos(b\cdot t)+da⋅cos(b⋅t)+da, dot, cosine, left parenthesis, b, dot, t, right parenthesis, plus, d. At t=0t=0t, equals, 0, when she starts playing, the accordion is 15\text{ cm}15 cm15, start text, space, c, m, end text long, which is the shortest it gets. 1.51.51, point, 5 seconds later the accordion is at its average length of 21\text{ cm}21 cm21, start text, space, c, m, end text. Find A(t)A(t)A, left parenthesis, t, right parenthesis. \textit{t}tstart text, t, end text should be in radians.