You have:
sin 2A = -√7/4
In order to determine the value of sin A, first calculate the value of angle A by using sin⁻¹ over the previous equation, just as follow:
sin⁻¹(sin 2A) = sin⁻¹(-√7/4) In this way you cancel out the sin
2A = -41.41° divide by 2 both sides
A = -41.41°/2
A = -20.705°
however, take into account that angle A is in the third quadrant. Then, it is necessary to consider the result A=-20.705° is respect to the negative x-axis.
To obtain the angle respect the positive x-axis (the normal way), you simply sum 180° to 20.705°:
20.705 + 180° = 200.705°
Next, use calculator to calculate sinA:
sin(200.705°) = -0.3535