Can someone explain to me how to find the eqaution?

To get the equation of any line you need two things: 
the slope of the line and any point passes by the line 

here we have two points passes by the line ( 0,2) (4,0) 
by knowing those two points we can get the slope easily as follow 

slope (m) = (0-2) / (4-0) 
so m = -1/2 
now we have the slope and point (0,2) 
the general equation of the line is y = mx +c 
where m is the slope and c is a constant. 
so the equation will be y = -1/2 x + c 
and to get the value of the constant, sub by the point (0,2) in the pervious equation

that will be 2 = -1/2 (0) + c 
then c = 2 

now the final equation of the line will be y = -1/2 x + 2 
to make the equation look better, multiply it by 2 and move x to the left-hand side 
2y + x = 4