Which is the correct algebraic expression after combining like terms? 6 + 8 x minus 7 minus x 7 x minus 1 7 x + 13 9 x minus 1 9 x + 13

1. ∠1 is complementary to ∠2 and ∠3 is complementary to ∠2. subtraction property of equality 2. m∠1 + m∠2 = 90 and m∠3 + m∠2 = 90 substitution 3. m∠1 + m∠2 = m∠3 + m∠2 definition of complementary 4. m∠1 = m∠3 given