(08.05 hc) the work of a student to solve a set of equations is shown: equation a: y = 15 − 2z equation b: 3y = 3 − 4z step 1: −3(y) = −3(15 − 2z) [equation a is multiplied by −3.] 3y = 3 − 4z [equation b] step 2: −3y = 15 − 2z [equation a in step 1 is simplified.] 3y = 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?