Answer:
Please check the explanation.
Step-by-step explanation:
Given
f(x) = 3x + x³
Taking differentiate



solving






now solving




Thus, the expression becomes


Thus,
f'(x) = 3 + 3x²
Given that f'(x) = 15
substituting the value f'(x) = 15 in f'(x) = 3 + 3x²
f'(x) = 3 + 3x²
15 = 3 + 3x²
switch sides
3 + 3x² = 15
3x² = 15-3
3x² = 12
Divide both sides by 3
x² = 4



Thus, the value of x will be:

C is the answer.............
Answer: The largest angle is 7x
Step-by-step explanation:
Their sum is 180, so
3x + 5x + 7x = 180
5x = 180 --> x = 12
The largest angle is 7x, which is 7(12) = 84