Question 511720
find the whole number such that four times the number subtracted from three times the square of the number makes 15
.
Let x = the number
then
3x^2 - 4x = 15
3x^2 - 4x - 15 = 0
replace -4x with:
3x^2 - 9x + 5x - 15 = 0
group terms:
(3x^2 - 9x) + (5x - 15) = 0
factor:
3x(x - 3) + 5(x - 3) = 0
(x - 3)(3x + 5) = 0
x = {3, -5/3}
throw out the fraction since we're looking for whole numbers
x = 3