We are given 2 points with there coordinates easy to calculate:
the upper one A(1,15) & the lower one B(6,10). Moreover the equation of this linear function is y = mx + b
1st) calculate m= (10-15) / (6-1) [FOMULA m=(y₂ - y₁) / (x₂ - x₁)]
m= -5/5 ==> m= - 1==> y=-x + b. To calculate b, replace x & y by any of the 2 points A, B. Say A(1,15)
15 = - (1) + b ==> b= 16 & final equation y = -x +16
