First step is to do distributive property.
so you multiply 5 by -3.4k and -7
your equation now is -17k-35+(3k+21)
Then you add like terms
add -17k+3k to give you -14k-35+21
then you add like terms again
add -35+21 to give you -14
so now your equation is -14k-14
So that is your answer simplified