ANSWER
[tex] \frac{1}{2} [/tex]
EXPLANATION
The given expression is;
[tex]2 {x}^{ - 4} y[/tex]
We want to find the value of this expression when x=2 and y=4.
We substitute to get,
[tex]2 {(2)}^{ - 4} (4) [/tex]
[tex]8 \times {2}^{ - 4} [/tex]
Recall that
[tex] {a}^{ - m} = \frac{1}{ {a}^{m} } [/tex]
This implies that,
[tex]8 \times {2}^{ - 4} = 8 \times \frac{1}{ {2}^{4} } [/tex]
[tex]8 \times {2}^{ - 4} = 8 \times \frac{1}{16 } = \frac{1}{2} [/tex]
