When combining like terms, it means that we can add or substract the terms that have the same literal part and simplify it as a single number.
In this case
[tex]7x^4-5x^{4^{}}[/tex]we can see that both expressions have the same literal part of x to the forth power, which means we can take it as a common factor
[tex]x^4(7-5)[/tex]this can be understood as subtracting 7-5 and leaving the same literal part
[tex]2x^4[/tex]