displacement from the t axis is 7cm when t is 0 amplitude is

displacement from the t axis is 7cm when t is 0, amplitude is 7 cm and period is 0.4 seec

Let the trigonmetric function be y = Asin(bt +c)

amplitude , a = 7cm

period , t =0.4sec

b = 2pi/0.4 = 5pi

y = 7sin(5pi*t +c)

at t=0 ; y =7cm

So, 7 = 7 sin(c)

c = pi/2

So, y = 7sin(5pi*t +pi/2) = 7cos(5pi*t)

Equation : y =7cos(5pi*t)