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: