The equation you are thinking of is for a trapezoid or parallelogram. Rectangles have 4-90 degree angles and two different side lengths, so unless the picture is drawn to scale and you have a ruler, you cannot figure out the "height," or adjacent side of the rectangle. You may be able to figure it out by drawing a diagonal line through the rectangle and using sine/cosine/tangent to figure out the sides and angles, but it doesn't seem like you have enough information to do so. However, area of a rectangle is determined by the following equation: Length x Width = Area. So if you know the length (base) and the area, you can simply divide the area by the base and you will most likely have the answer.
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Multiply them by eachother.
If it is a rectangle then area = base × height = 7cm × 12 cm = 84 cm². If it is a triangle (more likely given the terminology used) then area = ½ base × height = ½ × 7 cm × 12 cm = 42 cm²
If you draw a diagonal in a rectangle you get two equal triangles, each half the area of the rectangle. Area of rectangle is base x height, so half of that is ½ x base x height. QED
Divide the area by the height
The one alternative to find the area of a rectangle is when you are given the length of one diagonal and its slope.