For 8m-(8-m)=28, you have to distribute the negative outside the parentheses into the 8 and the -m.
-1 times 8 = -8, and -1 times -m = m. So, you have 8m-8+m = 28. Can you take it from here? (Hint: Collect like terms)
For the second equation 8x-5=15-x, add x to both sides. 8x-5+x=15-x+x. Simplify and you get 9x-5=15. Do you understand those steps?