First solve for the slope, m using the two points given. It doesn't matter which point you choose as point 1 or 2 as long as you're consistent.
m = (y2 - y1)/(x2 - x1)
point 1: (6.4, 2.6)
point 2: (5.2, 9)
m = (9 + 2.6)/(5.2 + 6.4)
m = (9 + 2.6)/(5.2 + 6.4)
m = 11.6/11.6
m = 1
put the newly found slope into the linear equation for m
y = (1)x + b
y = x + b
Now solve for the y-intercept, b
by putting one of the given points
9 = 5.2 + b
b = 9 - 5.2
b = 3.8
final equation:
y = x + 3.8