Why do this problem?
This tangram is a challenging context in which to remind pupils of the properties of squares.
If preferred, they could print off this sheet
and cut out the tangram pieces rather than using the interactivity.
In both cases, it is important that children become more confident at simply "having a go" and playing with their ideas to begin with, rather than feeling that they need to know the "right" way to do it from the start. You might like to model this trial and improvement approach, perhaps on an interactive whiteboard, emphasising that we can learn a lot from what doesn't work.