I am having stuck answering this question Watch
remember that economic growth is shown by an increase in real GDP. GDP can be measure by either total output, income or expenditure in an economy, but we tend to measure by output (however, all of these should be equal anyway considering the circular flow model which i assume you've learned?)
Think back to factors that affect aggregate demand. Net exports are one of these factors. As exports increase and/or imports decrease, this causes an increase in AD. AD increase results in increased output (as it will cause an extension in aggregate supply) and therefore there will be increased output-> therefore economic growth.
It all comes down to linking exports with AD which will affect GDP hope this helps!