Let's just quickly note that the area of a triangle is base*height/2
So let's use XY as the base and use Z for the height
The base, or length of XY is 9 because the x values are the same(-5) and the y values are 9 apart(-1-(-10)=9)
Thus we now know our base is 9.
Now let's figure out the height.
In the graph, draw a perpendicular line from Z to line XY. You will find that the length of the line drawn is actually the change in X from Z to X or Y.
Thus, the change of X between -9 and -5 is 4 because -5-(-9)=4.
Before I continue, I want to note that it could be -9-(-5)=-4 but it really doesn't matter because we are trying to find the distance between the tho points and distance can't be negative.
Now to continue:
We have the height, which is 4 units, and we have the length, which is 9 units. Thus we plug this into our formula to get A=H*B/2=4*9/2=18 units^2