If x = 4cm find the area of this triangle

To find the area of a given triangle, we multiply the base by the height, and then divide the result by 2.
So, we use the following formula:
Area=height * base / 2
In short;
A=hb/2

As for your question,
if x=4;

A=7x*8x/2
A=7(4)*8(4)/2
A=28*32/2
A=896/2
A=448 (cm/sq)

So, the answer is D.