There are a number of questions like this .... normally use of FIG 4.7.3 would involve you working through the chart from DISTANCE to WIND to ALTITUDE to WEIGHT and arriving at the FUEL required.
For this question, the requirement is to find the ALTITUDE. To come up with the answer you need to work normally through DISTANCE and WIND, and then backwards through FUEL and WEIGHT.
Start at 950nm...to the WIND REF LINE...adjust for 20kt Head. Draw a line straight up through the ALTITUDE section.
Now go to the FUEL scale 5800kg ...we need to work backwards ... (backwards use of this ref lines needs you to go from 5800kg to the 500000kg condition, then adjust back to the reference line). Luckily in this example, the weight condition of 50000kg is the ref line value, hence there is no adjustment required - all we have to do is draw a line from 5800kg straight through the ref line and through the altitude section. Your 2 lines should meet at 25000ft.