Make sure the triangle is properly inscribed on (inside and touching the sides of) the rectangle and redraw it if it is not. The bottom edge of the triangle should form the bottom edge of the rectangle so that the bottom corners (vertices) of the triangle are located exactly on the corners of the rectangle. The top angle of the triangle should be somewhere on the top edge of the rectangle. Hint: If this is not the case, redraw the rectangle around the triangle by using the bottom edge of the triangle as the rectangle base, drawing two lines vertically up from the two end points of this bottom edge and connecting the endpoints of these two vertical lines with a horizontal line that passes through the top angle of the triangle.
Find the length value of the bottom edge of the rectangle (which is also the base of your triangle). Note this value as B.
Find the length value of the vertical edges of the rectangle (this is the height of your rectangle and therefore also the height of your triangle). Note this value as H.
Multiply B from Step 2 and H from Step 3 together and divide this by 2. This is the area in units squared of the remaining triangle.