Hi! hope this helped
B(d) is a function of the number of days d since she opened the account.B(0) is the balance at day=0 now, since the length of segment OA is half of the length of segment OB, and the length of segment OB is proportional to the max balance, namely $400, then the segment OA is proportional to 400/2=200
The values of sin θ and cos θ represent the legs of a right triangle with a hypotenuse of 1; therefore, solving for cos θ finds the unknown leg, and then all other trigonometric values can be found.