Take into account that the general form of an equation of a line is given by:
y = mx + b
where m is the slope of the line and b the y-intercept.
Calculate the slope of the line as follow:
m = (y2 - y1)/(x2 - x1)
where (x1,y1) and (x2,y2) are any two points of the line.
Use
(x1,y1) = (0,30)
(x2,y2) = (50,60)
replace the previous values of the parameters into the equation for m:
m = (60 - 30)/(50 - 0)
m = 30/50
m = 3/5
the y-intercept is the value of y when x=0. On the graph you can notice that such value is y = 30. Then, b = 30.
Replace the values of m and b into the general equation of a line.
y = 3/5 x + 30
th