The area of a triangle is given by the formula ...
A = 1/2bh
__
This triangle has a base of 4 units and a height of 5 units. (These dimensions can be counted from the graph, or found by subtracting coordinate values.) Its area is ...
A = 1/2(4 units)(5 units) = 10 square units
_____
Additional comment
The first two coordinates lie on the line x=0. The length of that segment is the difference of the y-values: 4 -(-1) = 5.
The first and last coordinates lie on the line y=-1. The length of that segment is the difference of the x-values: 4 -0 = 4.
