Answer:
Step-by-step explanation:
Count the squares. The horizontal extent is 3 squares, so is 3 units.
The vertical extent is 7 squares, so is 7 units. We're only concerned with the vertical distance between the horizontal lines at the top and bottom of the figure. The slant distance along the side of the figure is irrelevant.
From there, fill the numbers into the blanks and do the multiplication. "bh" means multiply the Base value by the Height value.
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Comment on units
It is not quite true that 3·7 = 21 units². What is true is that ...
(3 units)·(7 units) = 21 units²
The equal sign should be treated with respect and care. What is found on one side must always be strictly equal to what is found on the other side. You cannot simply add "units²" to one side of the equation because you feel like it. There must be something corresponding on the other side of the equation.
You can actually multiply and divide units (and in Physics, you're expected to do so). The units can be attached to the numbers and treated in the math as though they were a variable. This can be especially helpful in area, volume, time/speed/distance, and unit-rate problems.