I presume that this is talking about simplification with the rule of PEMDAS. First, look at the parenthesis and brackets. Let's do the [(7t + 5) - (8 - 3t)] part first. The parenthesis can go away on the inside if you distribute the negative in (8 - 3t), which flips the signs of those. You should then have [7t + 5 - 8 + 3t], then work it out to form just (10t - 3). The negative will also carry for the brackets that were there before, so - (10t - 3) turns into -10t + 3. Now, you just have 5t + t - 3 - 10t + 3. Work that out to get your final answer of -4t.
-4t is the answer.