Question

For the system of equations shown, tell whether it would take fewer steps to solve by substitution or elimination. Then use that strategy to solve the system. Explain your reasoning.

-3x - 4y = 15
9x + 7y = 25

Answer 1

Explanation:

Substitution is a huge headache. You with be working with double digit numbers before you can eliminate one of the variable.

The best way to do this is to multiply the top equation by 3

3(-3x - 4y = 15)

-9x - 12y =45 Now add this and the second equation together.

9x + 7y = 25 The xs disappear. They are eliminated,

-5y = 70 Divide by -5

y= - 14

Now use the original top equation to find x

-3x - 4y = 15

-3x - 4(-14) = 15

-3x + 56 =15 Subtract 56 from both sides.

-3x = 15 - 56

-3x = - 41 Divide by - 3

x = - 41/-3

x = 13.6667