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?
a: step1
b: step 2
c: step 3
d: step 4