Answer:
About 76.159 square feet
Step-by-step explanation:
This can be solved by using an equation. Let's call the combined width of the sandbox and grass x+5, and the length x+7.
(x+5)(x+7)=189
x^2+12x+35=189
x^2+12x-154=0
x≈7.784 feet
x(x+2)≈76.159. Hope this helps!