z=3k+i+1/10(12+k)+(3k-4)i

$$\\z=3k+i+\frac{1}{10}*(12+k)+(3k-4)i\\\\
z=3k+i+\frac{12+k}{10}+(3k-4)i\\\\
z=\frac{30k}{10}+\frac{12+k}{10}+(3k-4)i+i\\\\
z=\frac{12+31k}{10}+(3k-3)i\\\\$$

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More than likely you're being asked to combine like terms and write a simplified answer. So put all the k terms together and the I terms to gether and let's see what we get

z=3k+i+1/10(12+k)+(3k-4)i

=3k+i+12/10+k/10+3ki-4i

=3k+3ki+12/10+k/10-3i

31k/10+3ki-3i+6/5