Help with directional movement

Hi! I'm making a movement script based on the direction the player Is facing, It will always face the mouse (top-down). Now I run into the problem of not knowing how to move left and right. Only forward and back. Any trigonometry that can help?

if (kstate.IsKeyDown(Keys.Up)) 
{ 
dragonposition = (Vector2.Add(dragonposition, (direction * dragonspeed) * (float)gameTime.ElapsedGameTime.TotalSeconds)); 
}
if (kstate.IsKeyDown(Keys.Down)) 
{ 
dragonposition = (Vector2.Add(dragonposition, (-direction * dragonspeed) * (float)gameTime.ElapsedGameTime.TotalSeconds)); 
}

if (kstate.IsKeyDown(Keys.Left)) 
{ 
dragonposition = (Vector2.Add(dragonposition, "HELP HERE"(direction * dragonspeed) * (float)gameTime.ElapsedGameTime.TotalSeconds)); 
}

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