Here's how I would do this. (Most of the time there are multiple ways to do a proof so this is not the one and only correct proof:
It's for a proof.
Given: j || k, m is perpendicular to j
Prove: m is perpendicular to k
Statements------------------------------------------Reason
1. m is perpendicular to j--------------------------1. Given
2. Angle 1 is a right angle.........................2. Def. of perp. lines
3. m of Angle 1 = 90 degrees........................3. Def. of right angles
4. j || k-------------------------------------------4. Given
5. angle 1 is congruent to angle 2------------------5. Alternate interior angles
6. m of angle 1 = m of angle 2......................6. Def. of congruent angles
7. 90 = m of angle 2................................7. Substitution property of equality
(Substituting the 90 from statement 3)
8. angle 2 is a right angle.........................8. Def. of right angles
9. m is perpendicular to k--------------------------9. Def. of perp. lines