Solve the following system of equations by linear combination:

2d - e = 7

d + e = 5

If you add them straight down:

2d-e = 7

d + e = 5

Then the e's cancel out.

You are left with 3d = 12

Then solve for d by dividing each side by 3.

Now you have d = 4.

To solve for e go back into one of your original equations and substitute 4 for the d.

You can use either equation, so I'll choose the second one.

4 + e = 5

Now solve for e by subtracting each side by 4.

Now you have e = 1.

So your solutions are d = 4 and e = 1