Trying to calculate in MB the space being taken across 3 database tables??

hello,
I have 3 tables in my sql database
Images
Audio
Video
I want to calculate how much space in MB  is taken by the content in all 3 tables by ArtistId parameter.
the code below works for images table but I want to add the audio and video to the select statement.
I tried using AND but it got an error??

Page Load
Guid ModelId = new Guid();

// UserId is a valid GUID format so assign it to a GUID variable ModelId = new Guid(Request.QueryString["ArtistId"]);

string strSQL = "SELECT [FilePath] FROM [Images] WHERE ([ArtistId] = @ArtistId)";
SqlConnection conn = new SqlConnection("Data Source=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"); conn.Open();
SqlCommand comm = new SqlCommand(strSQL, conn); comm.Parameters.Add("@ArtistId", ...