Can someone help me write a formula for y = f(t)?

The amplitude is (5 - 1) ÷ 2 = 2 so you know it starts out with a 2 in front.

The midline is (5 + 1) ÷ 2 = 3 so you know there is a + 3 at the end.

The period is 24 making the frequency be 2π/24 = π/12 so that is multiplied by t

and the midpoint on the way up hits when t = 1 (max is 7, minus 6 hours for a quarter of a cycle) so 1 is subtracted from t (in parentheses, before multiplying by π/12)

y = 2 sin [π/12 (t - 1)] + 3

IMO anyway, try it and see if it makes sense

general equation has form

y=A*sin(omega*t+phase)+offset

high and low peaks are 5 and 1m, from there we can determine that peak-to-peak is 5-1=4m which is two amplitudes (positive and negative), therefore amplitude is half of that or A=2 meter

midline is in center so it is 3m (offset = 3) because (5+1)/2=3, that is permanent offset (usually shown as added term at the end)

period is

T=1/f=24 (hours)

omega = 2*pi*f = 2*pi/T = 6.28/24 = 0.261666 rad/h

sin(x) is max when x=pi/2 which is at 7am (when t=7)

pi/2 = 7*2*pi/24 + phase

phase = pi/2 - 7*pi/12 = 6*pi/12 - 7*pi/12 = -pi/12

therefore

y(t) = 2*sin[(pi/12)*t -pi/12] + 3

or

y(t) = 2*sin[(t-1)*pi/12] + 3, where t is in hours