Answer: The required answer is 3x².
Step-by-step explanation: We are given that the sum of an expression E and 7x² is 10x².
We are to find the value of the expression E.
According to the given information, we have
[tex]E+7x^2=10x^2\\\\\Rightarrow E=10x^2-7x^2\\\\\Rightarrow E=(10-7)x^2\\\\\Rightarrow E=3x^2.[/tex]
Thus, the required answer is 3x².