Show work and help PLEASE 5 6 7 and 8 please

5. -10y+18=-3(5y-7)+5y
do the parenthasees first
-10y+18=-15y+21+5y
group like terms (y with y and numbers with numbers)

-10y+18=-10y+21
add 10y to both sides and you get
18=21 which is false
so no solution

6. 8(t+2)-3(t-4)=6(t-7)+8
do the parenthasees
8t+16-3t+12=6t-42+8
group like terms
5t+28=6t-36
subtract 5t from both sides
28=t-36
add 36 to both sides
64=t

7.  do the parenthasees first
20+8x-5=9x+21
add like terms
15+8x=9x+21
subtract 8x from both sids
15=x+21
subtract 21 from both sides
x=-6

8. distribute the parenthasees 
12x-48+3=15
add like terms
12x-45=15
add 45 to both sides
12x=60 
divide both sides by 12 
x=5

It is important to remember that

X^(m/n) = m✔️X^n.

It would be C, I believe.