One method to see it's a trapezoid is to find the slope of lines BC and AD.
The slope formula is
m = (y2-y1)/(x2-x1)
You should find that BC and AD both have the same slope (of -1), so that means the lines are parallel. That proves we have a trapezoid.
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To prove this trapezoid is isosceles, you can use the distance formula
to find the lengths of AB and CD (the two non-parallel sides). You should find that AB = CD.
Because AB and CD are horizontal and vertical respectively, this means you can simply count out the spaces to find that AB and CD are 3 units each. For any other rotated version of this trapezoid, use the distance formula instead.