an equivalent expression means that we need an equation that is equal to the one you have listed
one way you can do this is to multiply the 7 by each term in the parentheses this will give us 14x + 63
Here is how we check if these are equivalent. pick a number to replace "x" and then solve the problem. if we get the same answer from both expressions then we are correct
Im going to choose the number 1 7(2(1) + 9) 7(2+9) 7(11) 77
now replace "x" with the number 1 in our new expression and if we get 77 then they are equivalent 14x+63 14(1)+63 14+63 77
