Since the x-coordinate of R is 7, the y-coordinate of R is equal to -4.
In Mathematics and Geometry, the point-slope form of a straight line can be calculated by using the following mathematical equation (formula):

Where:
- x and y represent the data points.
- m represent the slope.
First of all, we would determine the slope of line PQ by using these points (5,-12) and (15,-6);

Slope (m) = (-6 + 12)/(15 - 5)
Slope (m) = 6/10
Slope (m) = 3/5
At data point (5, -12) and a slope of 3/5, an equation for line PQ can be calculated by using the point-slope form as follows:
y + 12 = 3/5(x - 5)
y = 3/5(x) - 15
For the midpoint of PQ, we have:
Midpoint PQ =

Midpoint PQ = (10, -9)
Since the lines are perpeniduclar, the slope (m) of line l is given by;
m × 3/5 = -1
m = -5/3
At midpoint (10, -9) and a slope of -5/3, an equation for line l can be calculated by using the point-slope form as follows:
y + 9 = -5/3(x - 10)
y = -5/3(x) + 50/3 - 9
y = -5/3(x) + 23/3
When x is 7, the y-coordinate of R is given by;
y = -5/3(7) + 23/3
y = -35/3 + 23/3
y = -12/3
y = -4
Complete Question:
The points P and Q have coordinates (5,-12) and (15,-6) respectively. The point R lies on the line l, the perpendicular bisector of the line PQ. The x-coordinate of R is 7.
Find the y-coordinate of R.