The work of a student to solve a set of equations is shown:

equation a: y = 15 – 2z
equation b: 2y = 3 – 4z

step 1: –2(y) = –2(15 – 2z) [equation a is multiplied by –2.]
2y = 3 – 4z [equation b]
step 2: –2y = 15 – 2z [equation a in step 1 is simplified.]
2y = 3 – 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?

step1
step 2
step 3
step 4