As we know with any math problem, brackets always need to be sorted out first using BIDMAS. So first, we would expand the brackets.

#color(red)(-3)color(green)((t+color(red)(5))+color(green)((4t+color(red)(2))=color(red)(8)#

#color(red)(-3 xx color(green)(t)=color(green)(3t)#

#color(red)(-3 xx 5)=color(red)(-15#

As #color(green)((4tcolor(red)(+2))# cannot be expanded by anything is stays the same.

Removing the brackets:

#color(green)(-3t)color(red)(-15)+color(green)(4t)color(red)(+2)=color(red)(8)#

Collecting like-terms:

#color(green)(-3t+4t=t)#

#color(red)(-15+2=-13#

Plugging back into equation:

#color(green)(t)color(red)(-13)=color(red)(8)#

To get #color(green)(t)# on it's own, we #color(red)(+13)# to get rid of the #color(red)(-13)# as they cancel out. Remember we add #color(red)(13)# to both sides.

#color(red)(8+13=21)#

#therefore# #t=21#