Obtain an equivalent system by performing the stated elementary operation on the system. Replace the third equation by the sum of itself and 3 times the first equation. - 8x + 5y - 67 = 16 7x - 5y - 7z = -28 4x - 5y + 12z = -12 -8x + 5y - 67 = 16 + = 7x - 5y - 7z = -28 x = 4x - 5y + 12z = - 12 + = (Type integers or simplified fractions.)