The absolute value function negates its argument when its argument is negative. The expression 3x+12 will be negative when ...
3x +12 < 0
x +4 < 0
x < -4 . . . . . makes the absolute value function change its argument's sign
Using this fact, we can divide the domain into intervals below -4, at -4, and above -4. The above answer shows the function value in each domain interval.
The garden's width is 45. We know this because (length) 35x2 is 70 and you need to neutralize that by taking away 70 from both sides. Now divide by 2 on both sides to neutralize the 2 times x. meaning that leaves us with