4)

[tex]2 {}^{x} . 4 {}^{y} = \frac{1}{2} \\ 2 {}^{x} .2 {}^{2y} = 2 {}^{ - 1} \\ [/tex]

[tex]7 {}^{x + y} = 1 \\ 7 {}^{x + y} = 7 {}^{0} [/tex]

{x+2y=-1

{x+y=0 (-1)

y=-1

x+y=0

x-1=0

x=1