Question 621572
First look at how each solution is different from the required concentration.

The 28% is +4% from the required 24%
The 14% id -10% from the required 24%

So the effect of the 28% is only 4/10 of the effect of the 14% 

so you need 10/4 times more of the 28% than the 14%

So let us call the volume of the 14% 'x'
So the volume of the 28% is10x/4

As the total volume needed is 7 liters this may be written:

x + 10x/4 = 7

solve for x:
(multiply all by 4 to remove denominator of 10x/4)

4x + 10x = 28
14x = 28
x = 28/14
x = 2

So the volume of the 14% is 2 litres and therefore the amount of 28% is 10/4 x 2
=5

Check:
Multiply the effect of each concentration (i.e. the difference between the concentration and desired by the calculated volume needed and add together. If the desired concentration of 24% was reached the answer should be zero.

2 times -10 + 5 times +4 = 0