If a + b + c = 9 and ab + bc + ca = 18, then the value of $a^3 + b^3 + c^3 - 3abc$ is:

243

If x + y  = n

then, $x^3 + y^3$ = n³ - 3 × n × xy

If a + b + c = 9

ab + bc + ca = 18

Then the value of $a^3 + b^3 + c^3 - 3abc$ = ?

If the number of equations are less than the number of variables then we can put the extra variables according to our choice = 

So here two equations given and three variables are present so put c = 0

If a + b  = 9

ab = 18

Then the value of $a^3 + b^3$ =  9³ - 3 × 9 × 18

$a^3 + b^3$ = 729 - 486 = 243