Get the equation of the line containing PQ using the point-slope formula:
y - (-2) = 3/2 (x - (-6))
Solve for y to get it in slope-intercept form:
y = 3/2 x + 7
so the y-intercept is (0, 7).
The line containing QR is then
y - 7 = -3/4 (x - 0)
or
y = -3/4 x + 7
The point R is on the x-axis, so its y-coordinate is 0. Plug in y = 0 and solve for x to get the other coordinate:
0 = -3/4 x + 7
3/4 x = 7
x = 4/3×7 = 28/3
So the point R has coordinates (28/3, 0).