Can you help me with this it's synthetic substitution or direct substitution

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[Eqn 3]

f(x) = x³ + 3x² + 6x - 11 when x = 5

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[Eqn 4]

f(x)=x³ - x² + 12x - 15 when x = 1

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[Eqn 5]

f(x) = 2x³ + 6x² + 4x + 8 when x = 2

step-by-step explanation:

to find if any function is an inverse of the other, put the first equation in as x in the next one. then simplify and solve. if at the end you get the statement y = x, then it is an inverse. if you get anything else, it is not.