We can express the original area of a triangle as
a=1 ( B)( H)

If the new base is 10% greater, the new b is (1.1)b.
If the new height is 20% less, the new h is (0.8)h.

Now substituting, we get
a=1 (1.1 X B)(0.8 X H)
a=1 (0.88) BH

Notice how the original area is only 88% of what it was...
So there was a 12% decrease in area.