First, you need to find the 4th vertex. To find that, you need to find the slope from point (2,5) and point (0,-1).
Slope is rise over run or (y2-y1) over (x2-x1). The slope is 6/2. So from point (-5,-3) you need to count 2 spaces to the right of -5 which is equal to (-3). Count 6 spaces to the to of -3 which will be equal to (+3). So the 4h point/vertex is found on (-3,3).
To find the area use the matrix form. I'll attach a picture for you