change font of substring swift

let longestWordRange = (longString as NSString).range(of: longestWord)</p> <p>let attributedString = NSMutableAttributedString(string: longString, attributes: [NSAttributedStringKey.font : UIFont.systemFont(ofSize: 20)])</p> <p>attributedString.setAttributes([NSAttributedStringKey.font : UIFont.boldSystemFont(ofSize: 20), NSAttributedStringKey.foregroundColor : UIColor.red], range: longestWordRange)