Filling in the given value, you have
... (sin θ)² + (3/4)² = 1
... (sin θ)² = 1 - 9/16 = 7/16 . . . . . . . subtract the constant on the left
... sin θ = √(7/16) . . . . . . . . . . . . . . take the square root
... sin θ = (√7)/4
