Triangles IJK and LMN are similar. Then, the proportion between their similar sides must be equal. That is:
IJ/ML = JK/MN = IK/LN
where:
IJ = x
JK = 54
IK = y
MN = 42
ML = 28
NL = 35
In order to find the value of y, you consider the equation JK/MN = IK/LN, and solve for y, just as follow
JK/MN = IK/LN replace by the values
54/42 = y/35 multiply both sides by 35
(54/42)(35) = y
45 = y
Hence, y = 45
