How to slow down a Rigidbody2D

Hey I am trying for slowing my player down this is the code I’m useing:

Rigidbody2D rb2d;

void FixedUpdate () {
        rb2d = GetComponent<Rigidbody2D>();
        if (Input.GetKey(KeyCode.W)) {
            this.rb2d.AddForce(transform.up * _speed / 2);
        }
        if (Input.GetKey(KeyCode.A)) {
            transform.Rotate(Vector3.forward * 0.75f * _speed);
        }
        if (Input.GetKey(KeyCode.D)) {
            transform.Rotate(Vector3.forward * -0.75f * _speed);
        }
        if () {
            Debug.Log("DownKey");
        }
        this.rb2d.velocity = this.rb2d.velocity * 0.9;
    }

I get the error: error CS0019: Operator '*' cannot be applied to operands of type 'UnityEngine.Vector2' and 'double'

Try change 0.9 to 0.9f on the final line. You’re only able to multiply a Vector2 or 3 by a float, but in C# having a decimal - e.g. 0.9 - is treated as a double unless you append the f to make it a float.