1Formula for gradient of a straight line.
m = (y2 - y1)/ (x2 - x1)
2Gradient-intercept form of equation of straight line.
y = mx + c
3General form of equation of straight line.
ax + by + c = 0
4Point-gradient form of equation of straight line.
y - y1 = m(x - x1)
5Parallel lines.
Same gradient
6Perpendicular lines.
m1m2 = -1
m1 = -1/m2 (negative reciprocal)
7Point of intersection of 2 lines.
Solve equations simultaneously