The graph goes through (0, 0), so is a sine function (not a cosine function). The maximum is 1 and the initial slope is negative, so the factor A is -1.
The extreme value is reached when x = π/4. We know the extreme value of -1·sin(α) will be reached when α = π/2. Thus α = 2x, and the factor B is 2.
Your equation is
y = -1·sin(2x)