Differential equations
solve the differential equations.
dy = 4 − x
/
dx

Anti derivative of 4 is 4x
anti derivative of -x is -1/2 x^2

so  y = 4x - (1/2)x^2 + C
(can't forget constants when doing anti derivatives xD)

is this what you need?

*edit
Anti-derivative of a Constant [just a number] becomes  (constant)*x
for example  integral of 5 = 5x + C

Then there is a Power Rule for integrals
integral of (x)^n = 1/(n+1) *(x)^(n+1) + C
for example: integral of  x^2 = 1/(2+1) * (x)^(2+1) + C = (1/3)*x^3 + C

11/2.2 = 5 inches for the length 8.5/2.2= 3.86 inches for the width