7a is a direct answer (1/2 *bh)
Now for 7b,
From the given qualities, we have the two sides and height of the triangle. Now the larger triangle can be split into two smaller ones, from which we can then obtain the length of the side of the smaller triangle. This can then be used to evaluate the area of the smaller triangle.
As for the dotted triangle, it's area should be equal to the area of the before mentioned smaller triangle (that's my best guess). This is because, both of it is right triangles with a common hypotenuse. Now, we can apply 1/2 * bh in reverse to get h
This is my best guess, it might be wrong.