Which system is equivalent to

5x^2+6y^2=50

7x^2+2y^2=10

We can add, subtract, multiply and divide an equation by anything as long as we do the same operation to both sides of the equation so that we still get an equivalent expression. If we multiply 7 to each term of the first equation, we can also write the polynomial in an equivalent form:

35x² + 42y² = 350

If we multiply - 5 to the second equation, we have an equivalent expression

-35x² - 10y² = - 50