I assume that you like math and coding. += has no place in math, but since you are using the letter m, I can tell you are asking for a slope. So the slope / 3 += 13. I am assuming you might be in calculus and are attempting to take a derivative? This would mean that x is quadratic.
That's a big increase in slope. It appears to be 39. This would mean that the equation is \(y = 19.5 x^2 + bx + c\)
If, however, you are not, and the + just got in there randomly: simply multiply each side by 3.