Get the slope of the line using the two known points:
slope = (7 3/4 - 6 1/4) / (1 1/4 - 3/4) = (1 2/4) / (2/4) = 3
Use the point-slope formula for a line to get the equation of the line. You can use any of the known points and the slope computed above. For example,
y - 6 1/4 = 3 (x - 3/4) ⇒ y = 3x + 16/4 = 3x + 4
Then for some given value of x, the third point has y-coordinate 3x + 4.
"Which values" suggests there may be more to the question, namely some list of answer choices. If you have something like that, just check if the above condition is met.
