Where did i go wrong - connected particals a level maths mechanics

Looks like you did it a similar way, but have cos(30)=1/2 when youve combined the equations. There may be other typos, but its easy to check with the model solution as the numbers are similar and this one wasn't hard to spot? Also you could have cancelled g as soon as T was eliminated etc ...

As an observation, rather than taking T over in the original 2 equations, then combining, then subtracting / elmininating, it would be easier just to add the original two equations? There is no need to have everything = 0 as you want to eliminate T then put m on one side and constants on the other. Aim to get stuff done as simply/efficiently/correctly as possible.