A=base times height
A=10 times 7
A=70 cm²
150*5.5=825
If I'm right, the actual length of the border is 825 miles. Hope I helped!
Starting from

take the first derivative using the power and chain rules:


Now take the second derivative:


Optionally, you can condense the second derivative a bit by factoring out
, which gives



The equation for ten times the sum of half a number and 6 is 8 would be: 10(1/2n+6)=8