It can be a litle tricky at first. Some may try to parse the expression and find out the operators that has the highest priority, calculate them and later calculate operator with lower priority. But if parentheses and functions are added to the expression it will soon be very complicated to do the calculation. An easier way to calculate it is to convert the expression from infix notation to postfix notation. This may first look very strange but when we need to do some calculations on a computer this is much easier.
Stacks are something that programmers are very familiar with.
With a stack a postfix expression can be calculated easily. If we have a stack and are going to calculate this formula we parse it and do some work with our stack, suddenly the expression is calculated. We push this value also. Now we have two values at the stack. And some calculation needs to be done.