Valeria is playing with her accordion. the length of the accordion a(t)a(t)a, left parenthesis, t, right parenthesis (in \text{cm}cmc, m) 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, space, c, m 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, space, c, m.