Show a card in a card game

Hi inventors!
I want to show the back of a card 500ms, then show a random card question.

when the condition is true.

the program call the "MOSTRAR TARJETA" procedure and dont show me the back of the card.

the question is, how can i show first the back of card and then the question.

there is no such thing as "wait" because this will block the main thread...
therefore to "wait", use a clock component, timer interval = 500ms, and start the clock
then in the Clock.Timer event continue with your logic
see also Please Wait method by Scott
see also The model of event processing in App Inventor by Lyn