Question 106672
Word problems are tricky to understand. The easiest way to derive an equation is to go step by step.

the first equation you can find is simple.

y=x-4

where x is the larger number and y is the smaller one

so if y is the smaller number 10x y subtracting 3x the larger number x looks like this

10y-3x=51

so your two equations are below

y=x-4
10y-3x=51

because you have y isolated you can substitute it in for the second equation

10y-3x=51
10(x-4)-3x=51

expand the brackets

10x-40-3x=51

simplify

7x-40=51

bring the 40 to the other side

7x=91
divide by 7

x=13

because you have the larger number subtract 4 to find the smaller number

your two numbers are (13 and 9)