The slope of the graph is 1/4 rise/run and this is the gradient (m) of the graph. Because the gradient/slope is positive it means it is increasing from left to right.
The equation of a line is y = mx + b
Where m is the gradient/slope in this case 1/4 and b is the y-intercept in this case zero.
Based on that information we substitute it into the formula y = mx + b to get
y = 1/4 x as the equation of the line.
Hope that helped