Incorrect. Look at the chart. The variable x represents the amount of the 15% solution you will need. Since you need 50 liters total, the amount of the 25% solution you will need is $\displaystyle{50}-{x}\usepackage{cmbright}\Huge\$ . Also note that 15% is 0.15 as a decimal, and 25% is 0.25.

The amount of acid in solution 1 added to the amount of acid in solution 2 gives you the amount of acid in the mixture, so $\displaystyle{0.15}{\left({x}\right)}+{0.25}{\left({50}-{x}\right)}={10}\usepackage{cmbright}\Huge\$ .