Idon't know how to solve. i keep trying tho

Alright, so for this, you have to;
1st- 3(7-x), you would multiply the outside # with everything on the inside and get 21 - 3x
2nd- You compare like terms, in this case, 6x and 2x, in turn you get 8x
3rd- You rewrite the problem to be -- 8x + 7 + 21 -3x
4th - You compare like terms again, this time, with the 7 and 21, you would add them since they both are positive and you would then get 28 
~As for the rest, you would do 8x-3x and get 5x
Your final answer in proper formatting would be 5x+28