Answer x = 12 and y = 9
Step by step
First identify the equations
The sum of two numbers is 21
x + y = 21
Change to y intercept for ease of math
y = 21 - x ( call this equation a)
Three times the larger is equal to four times the smaller
3x = 4y (call this equation b)
Now substitute the value of y = 21 -x into equation b
3x = 4(21-x)
Distribute
3x = 84 - 4x
Now we want to isolate x by adding 4x to each side
3x + 4x = 84 -4x + 4x
Combine like terms
7x = 84
Now solve for x by dividing both sides by 7
x = 12
Now to find y
Using equation a from above
y = 21 - x
Substitute our x value of 12 into equation
y = 21 - 12
solve for y
y = 9
Now check your work
x = 12 and y = 9
The sum of two numbers is 21
12 + 9 = 21
Three times the larger (x) is equal to four times the smaller (y)
3x12 = 4x9
36 = 36
Problem solved!!