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:

The awnser is 4.2 I hope this helps
You need to use y=mx+c to answer this so basically
gradient/slope (m) is 1.5 and the y-intercept (c) is -2
Hope this helps :)
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