I agree with the most recent comments. This wasn't as hard to solve as others have made out, and is worth having a go at, without recourse to programming or even Excel. The key, as others have pointed out, is the south-east corner. There are only six possible solutions for 24 and only nine non-clashing possibilities for the pair of 24 and 15. Considering 19 yields 17 possible solutions, and considering 21 on the left reduces these to one. Given 13, Z must be small, and the rest is straighforward. On the right, there is only one solution for 21 that fits and doesn't begin with a T.
For those who are bemused by the algebraic logic, there are mnemonics to help. The best of these is BODMAS, which stands for Brackets, Order, Division & Multiplication, and Addition & Subtraction. [Some use PEDMAS, for Parentheses, Exponents ...] In other words do everything in brackets or parentheses first, then all exponents or roots ("order"), then all divisions & multiplications, and finally all additions & subtractions. Within each set of brackets/parentheses the BODMAS order follows. It would have been clearer (e.g. in 5) if Ruslan had used parentheses nested within brackets, rather than parentheses within parentheses; in that case, you do the parentheses first and then everything within the brackets, according to the usual order.
To take an example, in 13 do I+D and then multiply it by F (save 1); do S+T, then multiply it by A and divide it by Z (save 2); then cube Z (save 3); now do 1 - 2 + 3.
To appreciate how well constructed this puzzle is, try to replace 16 with a simpler single expression that yields the separate answers on the left and right, using the two different sets of solutions.