because we know that the triangles are similar, that means the lengths of the sides are proportional to one another. we can see that the length of the given side for the bigger triangle is 2 times the smaller triangle (20/2=10). that means the missing side is two times bigger, so it would be 12.
now we can find the area
1/2*base*height = 1/2 * 12 * 20 = 120
hope this helps!