To find the width of the poster, we can use the formula for the area of a rectangle, which is given by:
\[ \text{Area} = \text{Length} \times \text{Width} \]
Given that the area is 42 square units and the length is 7 units, we can set up the equation:
\[ 42 = 7 \times \text{Width} \]
To find the width, we divide both sides of the equation by 7:
\[ \text{Width} = \frac{42}{7} = 6 \text{ units} \]
Thus, the width of the poster is 6 units.