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