Hi. So for this question, u need to work backwards.
A=bh/2
137.5=Bx11/2
137.5=11b/2
137.5=5.5b
137.5/5.5=25
So the base would be 25 cm. To check, do the actual formula from the start.
A=bh/2
A=25x11/2
A=275/2
A=137.5! The answer is right! The base is 25 cm! Hope that helps! :)