A common mistake when solving equations is the following:

The equation: 2(x - 2) = x + 3
First step in solving: 2x - 2 = x + 3

Write a clear explanation of what error has been made. What 
could be done to avoid this error?

When removing parentheses using the distributive principle,
the outer multiplier must be multiplied by EVERY term
inside the parentheses, not just the first one only!

The mistake above is multiplying the outer multiplier 2
by the first term in the parentheses x ONLY and forgetting
to multiply the outer multiplier 2 by the second term, -2,
to get -4, but instead erroneously just leaving it as -2 
in the first step.

The correct first step in solving is: 2x - 4 = x + 3 

Edwin