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this is a law of exponents problem as. Anything with common bases and being multiplied, have the exponent law of adding the exponents. Let's break this problem apart accordingly.

First the x's

-x * x^3

-x^(1+3)

-x^4

Now the y's

y^7 * y

y^(7+1)

y^8.

Now multiply the two variables by 7

7(-x^4y^8)

-7x^4y^8

Thus this is your final answer. -7x^4y^8

Please let me know if you need any clarification. I'm always happy to answer your questions.