Stopping at the slope: forgetting to solve for b
The question asks for the y-intercept, you correctly compute the slope, and one of the answer choices IS that slope. Gift accepted, points lost.
Why it happens
Slope is the satisfying computation, so finishing it feels like finishing the problem. The second step (plug a point back in to find b) has no computational 'click' to remind you.
See the trap
A line passes through (2, 7) and (4, 13). What is its y-intercept?
✗ The trap: m = (13 − 7)/(4 − 2) = 3. Answer: 3.
✓ The fix: m = 3, then 7 = 3(2) + b → b = 1.
How to actually fix it
- Write y = mx + b as a checklist: two unknowns means two steps.
- After finding m, immediately substitute one point to get b.
- Re-read what the question asked before bubbling.
Made this mistake on a real question?
Practice Algebra questions here. Miss one and it is logged with the exact error, then brought back until you beat it.
Practice Algebra →FAQ
Any shortcut for b?
b = y₁ − m·x₁ in one line. But the real fix is the habit of asking 'did I answer the actual question?'
How do I stop repeating this mistake?
Reading the explanation isn't enough — your brain needs to beat the trap on a fresh question. Practice the skill in the kaoshen.co question bank: miss one and it is logged with the exact error, then brought back to you three days later and again after fourteen.