Hi there! I am learning English and I wonder how to read this maths expression in English.
This expression is difficult to say unambiguously in English (or probably any natural language); mathematical notation is an artificial language designed to be exact within a much more limited domain than is covered by natural language. There is nothing in English like the rule of construing mathematical notation called PEMDAS in US schools. Of course, any mathematical expression can be stated exactly in English if you are very wordy. For the relatively simple expression shown as your example, here goes a rendering that is probably about as concise as possible:
The fraction a squared minus twenty five all over a plus three plus the fraction one over a squared plus five a minus the fraction a plus five all over a squared minus three a.
That will probably be interpreted correctly by most numerate speakers of American English if you are careful to indicate that the numerators of two of the fractions are a difference and a sum respectively. If you want to be absolutely sure of being understood by someone with mathematical training, you can combine the terms fraction, sum, difference, numerator, denominator, minuend, and subtrahend to duplicate the mathematical notation exactly, but the result will be a long paragraph. For example, you can describe
the first fraction, which has a numerator that is a difference, with a minuend of a squared and a subtrahend of twenty five, and with a denominator that is the sum of a and three
Thus, you can describe exactly each fraction in detail that way and put them together as
The expression is a difference. The minuend of the entire expression is the sum of two fractions. The first fraction in that sum is __. The second fraction in that sum is __. The subtrahend of the entire expression is the fraction __.
To conclude, brief descriptions in English of anything more than very simple mathematical expressions have a material probability of being at least somewhat ambiguous. Use the universal language of mathematical notation if you want to avoid any possibility of ambiguity.