We could find the error first, but if we solve the equation, we can compare our answers to find the error. So,
7x + 12 + 3x = 8
10x + 12 = 8
10x = -4
x = -4/10
x = -2/5.
When we compare the two, we see that our second line is vastly different, and we can easily isolate the mistake, finding that it occurs when they disregard the variable in the 3x term and subtract 8 by 3 rather than combining the terms 3x and 7x together.