Answer-
Set of constraints to model the problem are,
[tex]12x+9y\geq 510\\y \leq 2x\\y \geq 25[/tex]
Solution-
Let us assume,
x = the number of lawns weeded by Gwen,
y = the number of dogs walked by Fabio.
Gwen charges $12 each time she weeds a yard and Fabio charges $9 each time he walks a dog,
[tex]\text{Earnings of Gwen} = 12x\\\text{Earnings of Fabio} = 9y\\\text{Total earnings of Gwen and Fabio} = 12x+9y[/tex]
They need at least $510 to purchase the new gaming station, means they need $510 or more than $510.
An equation for this situation will be,
[tex]12x+9y\geq 510[/tex]
The number of dog walked by Fabio has scheduled is no more than twice the number of yards Gwen has scheduled to weed, means y must be less than or equal to 2x.
An equation for this situation will be,
[tex]y \leq 2x[/tex]
Fabio will walk at least 25 dogs, means y must be greater than of equal to 25.
An equation for this situation will be,
[tex]y \geq 25[/tex]