Put "0" in the integer postions of this "if" statement.
if ((da >=10) && (db >=10))
{
digitalWrite(lp, HIGH);
digitalWrite(ln, LOW);
digitalWrite(rp, HIGH);
digitalWrite(rn, LOW);
Serial.println(" go straight");
Serial.println(da);
Serial.println(db);
}
If the either sensor output results as "0" you will not go straight.