This mistake Marta made was that she substituted for y in the same equation where she made y the subject of the formula.
The system of equations that Martha is trying to solve is a simultaneous equation which she tried to solve using the substitution method.
10x + y = 22 equation 1
2x + y = -2 equation 2
Step 1: Make y the subject of the formula in equation 1
y = 22 - 10x
Step 2: Substitute for y in equation 2
2x + 22 - 10x = - 2
Step 3: Combine similar terms
2x - 10x = -2 - 22
-8x = -24
Step 4: divide both sides by -8
x = 3
This mistake Marta made is in step 2. Instead of substituting for y in equation 2 she substituted for y in equation 1.
To learn more about simultaneous equations, please check: brainly.com/question/23589883?referrer=searchResults