The work of a student to solve a set of equations is shown: equation a: y = 16 – 2z equation b: 7y = 2 – 4z step 1: –7(y) = –7(16 – 2z) [equation a is multiplied by –7.] 7y = 2 – 4z [equation b] step 2: –7y = 16 – 2z [equation a in step 1 is simplified.] 7y = 2 – 4z [equation b] step 3: 0 = 18 – 6z [equations in step 2 are added.] step 4: 6z = 18 step 5: z = 3 in which step did the student first make an error?