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A pyramid with the top cut off.....I think you're talking about (technically) an isosceles trapezoid. Well, a rectangle is a good place to start
But it sounds like your rectangle has rounded corners. You can make them sharp by selecting it, and clicking this button
on the Rectangle
control barAfter that, do Path menu > Object to Path. That makes your rectangle
shape into a rectangular
path. Now you can edit with the Node tool
Click on one of the corner nodes, to select it. Then use the left or right arrow key to move it. Each time you press an arrow key, the selected node will move by 2 px. Hold the Shift key while you do it, and it will move 20 px each time. Hold the Alt key, and it will move something less than 2 px, and how much depends on the zoom level.
Or instead of arrow keys, you can use the X field on the Node tool control bar to move by some particular value.
Do the same thing for the other corner, only in the other direction.
This is one of the many things which can be done in several different ways, with Inkscape. Another way might be to start with the Star/Polygon tool. Use the Star/Polygon control bar to set to draw polygons, and set corners to 3. That will draw a triangle.
Hold the Ctrl key while you draw the 3-corner polygon (triangle), and drag straight up or down. That will make an isosceles triangle, which is the kind you need to end up with the isosceles trapezoid.
Then you can draw a rectangle and place it over the top of the triangle. Then select both, and do Path menu > Difference.
You can go to the Home tab of this website to find beginner tutorials. In Inkscape, Help menu > Inkscape manual. Also in Inkscape, and this is how I first learned to use Inkscape, Help menu > Tutorials > Basic, Shapes, and Advanced. Even though it's called "advanced" it really is for beginners too. There are other tutorials, but they are more for specific things.
