From the picture, the side which measures x+3 is congruent with the side which measures 15 units. Then,
x + 3 = 15
3 is adding on the left, then it will subtract on the right
x = 15 - 3
x = 12
From the picture, the side which measures 3y-1 is congruent with the side which measures 2x-4 units. Then,
3y - 1 = 2x + 4
Replacing with x = 12,
3y - 1 = 2(12) - 4
3y - 1 = 24 - 4
3y - 1 = 20
1 is subtracting on the left, then it will add on the right
3y = 20 + 1
3y = 21
3 is multiplying on the left, then it will divide on the right
y = 21/3
y = 7