What property is this ? (x+y)+z=x+(y+z)

Solve the equation 3x2 + 19x + 9 = 0 to the nearest tenth.

Which shows the correct substitution of the values a, b and c from the equation 1=-2x+3x^2+1

Jada has a meal in a restaurant she adds up the prices listed on the menu for everything they ordered and gets a subtotal of $42.00. when the check comes, it says they also need to pay $3.99 in sales tax. what percentage of the subtotal is the sales tax